Merge branch 'dm-project-group-share-docs' into 'master'

Clarify what groups projects can be shared with.

Closes #39115

See merge request gitlab-org/gitlab-ce!17309

doc/user/project/members/share_project_with_groups.md

@@ -16,19 +16,29 @@ say 'Project Acme', in GitLab is to make the 'Engineering' group the owner of 'P
 Acme'.  But what if 'Project Acme' already belongs to another group, say 'Open Source'?
 This is where the group sharing feature can be of use.
 
-To share 'Project Acme' with the 'Engineering' group, go to the project settings page for 'Project Acme' and use the left navigation menu to go to the **Settings > Members** section.
+To share 'Project Acme' with the 'Engineering' group:
 
-![share project with groups](img/share_project_with_groups.png)
+1. For 'Project Acme' use the left navigation menu to go to **Settings > Members**
 
-Then select the 'Share with group' tab by clicking it.
+    ![share project with groups](img/share_project_with_groups.png)
 
-Now you can add the 'Engineering' group with the maximum access level of your choice. Click 'Share' to share it.
+1. Select the 'Share with group' tab
+1. Add the 'Engineering' group with the maximum access level of your choice
+1. Click **Share** to share it
 
-![share project with groups tab](img/share_project_with_groups_tab.png)
+    ![share project with groups tab](img/share_project_with_groups_tab.png)
 
-After sharing 'Project Acme' with 'Engineering', the project will be listed on the group dashboard.
+1. After sharing 'Project Acme' with 'Engineering', the project will be listed
+   on the group dashboard
 
-!['Project Acme' is listed as a shared project for 'Engineering'](img/other_group_sees_shared_project.png)
+    !['Project Acme' is listed as a shared project for 'Engineering'](img/other_group_sees_shared_project.png)
+
+Note that you can only share a project with:
+
+- groups for which you have an explicitly defined membership
+- groups that contain a nested subgroup or project for which you have an explicitly defined role
+
+Admins are able to share projects with any group in the system.
 
 ## Maximum access level