Add documentation page for bulk editing group issue milestones

- Add new page to group docs for bulk editing issues
- Add link to new section to group index
- Amend bulk editing page at a project level to remove issues from note

doc/user/group/bulk_editing/index.md

+# Bulk editing issue milestones **(PREMIUM)**
+
+> [Introduced](https://gitlab.com/gitlab-org/gitlab-ee/issues/7249) in
+[GitLab Ultimate](https://about.gitlab.com/pricing/) 12.1.
+
+NOTE: **Note:**
+A permission level of `Reporter` or higher is required in order to manage issues.
+
+Milestones can be updated simultaneously across multiple issues by using the bulk editing feature.
+
+![Bulk editing](img/bulk-editing.png)
+
+To bulk update group issue milestones:
+
+1. Navigate to the issues list.
+1. Click **Edit issues**.
+    - This will open a sidebar on the right-hand side of your screen where an editable field
+  for milestones will be displayed.
+    - Checkboxes will also appear beside each issue.
+1. Check the checkbox beside each issue to be edited.
+1. Select the desired milestone from the sidebar.
+1. Click **Update all**.

doc/user/group/index.md

@@ -78,6 +78,10 @@ Issues and merge requests are part of projects. For a given group, you can view
 [issues](../project/issues/index.md#issues-list) and [merge requests](../project/merge_requests/index.md#merge-requests-per-group) across all projects in that group,
 together in a single list view.
+### Bulk editing issues
+
+For details, see [bulk editing issues](../group/bulk_editing/index.md).
+
 ## Create a new group
 > For a list of words that are not allowed to be used as group names see the

doc/user/project/bulk_editing.md

@@ -13,13 +13,20 @@ by using the bulk editing feature.
 ![Bulk editing](img/bulk-editing.png)
 NOTE: **Note:**
-Bulk editing of issues and merge requests is only available at the project level.
-To update multiple project issues or merge requests at the same time, navigate to
-their respective lists and click **Edit issues** or **Edit merge requests** available
-in the tab bar. This will open a sidebar on the right-hand side of your screen
-where editable fields will be displayed. Checkboxes will also appear to the left-hand
-side of eachissue or merge request for you to select the items you want to update.
-Once you have selected all relevant items, choose the appropriate fields and their
-values from the sidebar and click **Update all** to apply your changes.
+Bulk editing of merge requests is only available at the project level.
+For more details, see [bulk editing group issues](../group/bulk_editing/index.md).
+To update multiple project issues or merge requests at the same time:
+1. Navigate to their respective list.
+
+1. Click **Edit issues** or **Edit merge requests**.
+
+    - This will open a sidebar on the right-hand side of your screen
+    where editable fields will be displayed.
+
+    - Checkboxes will also appear beside each issue or merge request.
+
+1. Check the checkboxes of each items to be edited.
+1. Choose the appropriate fields and their values from the sidebar.
+1. Click **Update all**.