Add Info About Feature Flag Permissions Flag

Development on this has been halted for some time now, but this flag is
still around. I'm not sure if it is worth deleting all the code or not,
as we still have plans to develop the feature.

ee/config/feature_flags/development/feature_flag_permissions.yml

 ---
 name: feature_flag_permissions
-introduced_by_url: 
-rollout_issue_url: 
-group: 
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/10096
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/254981
+group: group::progressive delivery
 type: development
 default_enabled: false