Merge branch 'docs/issues-multiple-assignees' into 'master'

Add to docs: issues multiple assignees

Closes gitlab-ee#2440 and #32574

See merge request !11556

doc/user/project/issues/index.md

+# GitLab Issues Documentation
+
+The GitLab Issue Tracker is an advanced and complete tool
+for tracking the evolution of a new idea or the process
+of solving a problem.
+
+It allows you, your team, and your collaborators to share
+and discuss proposals, before and while implementing them.
+
+Issues and the GitLab Issue Tracker are available in all
+[GitLab Products](https://about.gitlab.com/products/) as
+part of the [GitLab Workflow](https://about.gitlab.com/2016/10/25/gitlab-workflow-an-overview/).
+
+## Use-Cases
+
+Issues can have endless applications. Just to exemplify, these are
+some cases for which creating issues are most used:
+
+- Discussing the implementation of a new idea
+- Submitting feature proposals
+- Asking questions
+- Reporting bugs and malfunction
+- Obtaining support
+- Elaborating new code implementations
+
+See also the blog post [Always start a discussion with an issue](https://about.gitlab.com/2016/03/03/start-with-an-issue/).
+
+## Issue Tracker
+
+The issue tracker is the collection of opened and closed issues created in a project.
+
+![Issue tracker](img/issue_tracker.png)
+
+Find the issue tracker by navigating to your **Project's Dashboard** > **Issues**.
+
+## GitLab Issues Functionalities
+
+The image bellow illustrates how an issue looks like:
+
+![Issue view](img/issues_main_view.png)
+
+Learn more about it on the [GitLab Issues Functionalities documentation](issues_functionalities.md).
+
+## New Issue
+
+Read through the [documentation on creating issues](create_new_issue.md).
+
+## Closing issues
+
+Read through the distinct ways to [close issues](closing_issues.md) on GitLab.
+
+## Create a merge request from an issue
+
+Learn more about it on the [GitLab Issues Functionalities documentation](issues_functionalities.md#18-new-merge-request).
+
+## Search for an issue
+
+Learn how to [find an issue](../../search/index.md) by searching for and filtering them.
+
+## Advanced features
+
+### Confidential Issues
+
+Whenever you want to keep the discussion presented in a
+issue within your team only, you can make that
+[issue confidential](confidential_issues.md). Even if your project
+is public, that issue will be preserved. The browser will
+respond with a 404 error whenever someone who is not a project
+member with at least [Reporter level](../../permissions.md#project) tries to
+access that issue's URL.
+
+Learn more about them on the [confidential issues documentation](confidential_issues.md).
+
+### Issue templates
+
+Create templates for every new issue. They will be available from
+the dropdown menu **Choose a template** when you create a new issue:
+
+![issue template](img/issue_template.png)
+
+Learn more about them on the [issue templates documentation](../../project/description_templates.md#creating-issue-templates).
+
+### Crosslinking issues
+
+Learn more about [crosslinking](crosslinking_issues.md) issues and merge requests.
+
+### GitLab Issue Board
+
+The [GitLab Issue Board](https://about.gitlab.com/features/issueboard/) is a way to
+enhance your workflow by organizing and prioritizing issues in GitLab.
+
+![Issue board](img/issue_board.png)
+
+Find GitLab Issue Boards by navigating to your **Project's Dashboard** > **Issues** > **Board**.
+
+Read through the documentation for [Issue Boards](../issue_board.md)
+to find out more about this feature.
+
+[Multiple Issue Boards](https://docs.gitlab.com/ee/user/project/issue_board.html#multiple-issue-boards)
+are available only in [GitLab Enterprise Edition](https://about.gitlab.com/gitlab-ee/).
+
+### Issue's API
+
+Read through the [API documentation](../../../api/issues.md).

doc/user/project/issues/issues_functionalities.md

+# GitLab Issues Functionalities
+
+Please read through the [GitLab Issue Documentation](index.md) for an overview on GitLab Issues.
+
+## Issues Functionalities
+
+The image bellow illustrates how an issue looks like:
+
+![Issue view](img/issues_main_view_numbered.jpg)
+
+You can find all the information on that issue on one screen.
+
+### Issue screen
+
+An issue starts with its status (open or closed), followed by its author,
+and includes many other functionalities, numbered on the image above to
+explain what they mean, one by one.
+
+Many of the elements of the issue screen refresh automatically, such as the title and description, when they are changed by another user.
+Comments and system notes also appear automatically in response to various actions and content updates.
+
+#### 1. New Issue, close issue, edit
+
+- New issue: create a new issue in the same project
+- Close issue: close this issue
+- Edit: edit the same fields available when you create an issue.
+
+#### 2. Todos
+
+- Add todo: add that issue to your [GitLab Todo](../../../workflow/todos.html) list
+- Mark done: mark that issue as done (reflects on the Todo list)
+
+#### 3. Assignee
+
+Whenever someone starts to work on an issue, it can be assigned
+to that person. The assignee can be changed as much as needed.
+The idea is that the assignee is responsible for that issue until
+it's reassigned to someone else to take it from there.
+
+> **Tip:**
+if a user is not member of that project, it can only be
+assigned to them if they created the issue themselves.
+
+##### 3.1. Multiple Assignees (EES/EEP)
+
+Issue Weights are only available in [GitLab Enterprise Edition](https://about.gitlab.com/gitlab-ee/).
+
+Often multiple people likely work on the same issue together,
+which can especially be difficult to track in large teams
+where there is shared ownership of an issue.
+
+In GitLab Enterprise Edition, you can also select multiple assignees
+to an issue.
+
+> **Note:**
+Multiple Assignees was [introduced](https://gitlab.com/gitlab-org/gitlab-ee/issues/1904)
+in [GitLab Enterprise Edition 9.2](https://about.gitlab.com/2017/05/22/gitlab-9-2-released/#multiple-assignees-for-issues).
+
+#### 4. Milestone
+
+- Select a [milestone](../milestones/index.md) to attribute that issue to.
+
+#### 5. Time Tracking (EES/EEP)
+
+This feature is available only in [GitLab Enterprise Edition](https://about.gitlab.com/gitlab-ee/).
+
+- Estimate time: add an estimate time in which the issue will be implemented
+- Spend: add the time spent on the implementation of that issue
+
+> **Note:**
+both estimate and spend times are set via [GitLab Slash Commands](../slash_commands.md).
+
+Learn more on the [Time Tracking documentation](https://docs.gitlab.com/ee/workflow/time_tracking.html).
+
+#### 6. Due date
+
+When you work on a tight schedule, and it's important to
+have a way to setup a deadline for implementations and for solving
+problems. This can be facilitated by the [due date](due_dates.md)). Due dates
+can be changed as many times as needed.
+
+#### 7. Labels
+
+Categorize issues by giving them [labels](../labels.md). They help to
+organize team's workflows, once they enable you to work with the
+[GitLab Issue Board](index.md#gitlab-issue-board).
+
+Group Labels, which allow you to use the same labels per
+group of projects, can be also given to issues. They work exactly the same,
+but they are immediately available to all projects in the group.
+
+> **Tip:**
+if the label doesn't exist yet, when you click **Edit**, it opens a dropdown menu from which you can select **Create new label**.
+
+#### 8. Weight (EES/EEP)
+
+Issue Weights are only available in [GitLab Enterprise Edition](https://about.gitlab.com/gitlab-ee/).
+
+- Attribute a weight (in a 0 to 9 range) to that issue. Easy to complete
+should weight 1 and very hard to complete should weight 9.
+
+Learn more on the [Issue Weight documentation](https://docs.gitlab.com/ee/workflow/issue_weight.html).
+
+#### 9. Participants
+
+- People involved in that issue (mentioned in the description or in the [discussion](../../discussions/index.md)).
+
+#### 10. Notifications
+
+- Subscribe: if you are not a participant of the discussion on that issue, but
+want to receive notifications on each new input, subscribe to it.
+- Unsubscribe: if you are receiving notifications on that issue but no
+longer want to receive them, unsubscribe to it.
+
+Read more on the [notifications documentation](../../../workflow/notifications.md#issue-merge-request-events).
+
+#### 11. Reference
+
+- A quick "copy to clipboard" button to that issue's reference, `foo/bar#xxx`, where `foo` is the `username` or `groupname`, `bar`
+is the `project-name`, and `xxx` is the issue number.
+
+#### 12. Title and description
+
+- Title: a plain text title describing the issue's subject.
+- Description: a text field which fully supports [GitLab Flavored Markdown](../../markdown.md#gitlab-flavored-markdown-gfm).
+
+#### 13. @mentions
+
+- Mentions: you can either `@mention` a user or a group present in your
+GitLab instance and they will be notified via todos and email, unless that
+person has disabled all notifications in their profile settings.
+
+To change your [notification settings](../../../workflow/notifications.md) navigate to
+**Profile Settings** > **Notifications** > **Global notification level**
+and choose your preferences from the dropdown menu.
+
+> **Tip:**
+Avoid mentioning `@all` in issues and merge requests,
+as it sends an email notification
+to all the members of that project's group, which can be
+interpreted as spam.
+
+#### 14. Related Merge Requests
+
+- Any merge requests mentioned in that issue's description
+or in the issue thread.
+
+#### 15. Award emoji
+
+- Award an emoji to that issue. 
+
+> **Tip:**
+Posting "+1" as comments in threads spam all
+participants of that issue. Awarding an emoji is a way to let them
+know you like it without spamming them.
+
+#### 16. Thread
+
+- Comments: collaborate to that issue by posting comments in its thread.
+These text fields also fully support
+[GitLab Flavored Markdown](../../markdown.md#gitlab-flavored-markdown-gfm).
+
+#### 17. Comment, start a discusion, or comment and close
+
+Once you wrote your comment, you can either:
+
+- Click "Comment" and your comment will be published.
+- Click "Start discussion": start a thread within that issue's thread to discuss specific points.
+- Click "Comment and close issue": post your comment and close that issue in one click.
+
+#### 18. New Merge Request
+
+- Create a new merge request (with a new source branch named after the issue) in one action.
+The merge request will automatically close that issue as soon as merged.
+- Optionally, you can just create a [new branch](../repository/web_editor.md#create-a-new-branch-from-an-issue)
+named after that issue.