Merge branch 'update-migration-docs' into 'master'

Update migration docs

See merge request gitlab-org/gitlab-ce!27261

doc/user/project/import/index.md

@@ -20,11 +20,28 @@ repository is too large the import can timeout.
 ## Migrating from self-hosted GitLab to GitLab.com
-You can copy your repos by changing the remote and pushing to the new server,
-but issues and merge requests can't be imported.
+If you only need to migrate git repos, you can [import each project by URL](repo_by_url.md), but issues and merge requests can't be imported.
 If you want to retain all metadata like issues and merge requests, you can use
-the [import/export feature](../settings/import_export.md).
+the [import/export feature](../settings/import_export.md) to export projects from self-hosted GitLab and import those projects into GitLab.com.
+
+NOTE: **Note:**
+This approach assumes all users from the self-hosted instance have already been migrated.
+If the users haven't been migrated yet, the user conducting the import
+will take the place of all references to the missing user(s).
+
+If you need to migrate all data over, you can leverage our [api](../../../api/README.md) to migrate from self-hosted to GitLab.com. 
+The order of assets to migrate from a self-hosted instance to GitLab is the following:
+
+1. [Users](../../../api/users.md)
+1. [Groups](../../../api/groups.md)
+1. [Projects](../../../api/projects.md)
+1. [Project variables](../../../api/project_level_variables.md)
+
+Keep in mind the limitations of the [import/export feature](../settings/import_export.md#exported-contents).
+
+You will still need to migrate your Container Registry over a series of
+Docker pulls and pushes and re-run any CI pipelines to retrieve any build artifacts.
 ## Migrating between two self-hosted GitLab instances
@@ -32,3 +49,6 @@ The best method for migrating a project from one GitLab instance to another,
 perhaps from an old server to a new server for example, is to
 [back up the project](../../../raketasks/backup_restore.md),
 then restore it on the new server.
+
+In the event of merging two GitLab instances together (for example, both instances have existing data on them and one can't be wiped), 
+refer to the instructions in [Migrating from self-hosted GitLab to GitLab.com](#migrating-from-self-hosted-gitlab-to-gitlabcom).