Specify that only project owners can transfer a project

[skip ci]

changelogs/unreleased/26651-cannot-move-project-into-group.yml

+---
+title: Specify in the documentation that only projects owners can transfer projects
+merge_request:
+author:

doc/workflow/groups.md

@@ -23,7 +23,7 @@ You can use the 'New project' button to add a project to the new group.
 
 ## Transferring an existing project into a group
 
-You can transfer an existing project into a group you own from the project settings page.
+You can transfer an existing project into a group you own from the project settings page. The option to transfer a project is only available if you are the Owner of the project.
 First scroll down to the 'Dangerous settings' and click 'Show them to me'.
 Now you can pick any of the groups you manage as the new namespace for the group.