Document unsupported subgroups feature setup

doc/user/project/code_owners.md

@@ -84,6 +84,10 @@ so that their members also become eligible Code Owners.
 If you do not invite **Subgroup Y** to **Project A**, but make them Code Owners, their approval
 of the merge request becomes optional.
 
+Inviting **Subgroup Y** to a parent group of **Project A**
+[is supported](https://gitlab.com/gitlab-org/gitlab/-/issues/288851). To set **Subgroup Y** as
+Code Owners, add this group directly to the project itself.
+
 ### Add a group as a Code Owner
 
 To set a group as a Code Owner: