Update permission docs. Add note about automatic unassigning when member leaves a team

doc/user/permissions.md

@@ -7,6 +7,9 @@ project itself, the highest permission level is used.
 On public and internal projects the Guest role is not enforced. All users will
 be able to create issues, leave comments, and pull or download the project code.
 
+When member leaves the team the all assigned Issues and Merge Requests
+will be unassigned automatically.
+
 GitLab administrators receive all permissions.
 
 To add or import a user, you can follow the [project users and members