Add feature flag note to group authorized agents docs

doc/user/clusters/agent/repository.md

@@ -152,6 +152,11 @@ gitops:
 
 > [Introduced](https://gitlab.com/groups/gitlab-org/-/epics/5784) in GitLab 14.3.
 
+FLAG:
+On self-managed GitLab, by default this feature is not available. To make it available,
+ask an administrator to [enable the `group_authorized_agents` flag](../../../administration/feature_flags.md).
+On GitLab.com, this feature is available.
+
 If you use the same cluster across multiple projects, you can set up the CI/CD Tunnel
 to grant the Agent access to one or more groups. This way, all the projects that belong
 to the authorized groups can access the same Agent. This enables you to save resources and