Document pipeline grouping
What does this MR do?
Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/28317 and https://gitlab.com/gitlab-org/gitlab-ce/issues/29176
Do not update/delete: Banner broadcast message test data
Do not update/delete: Notification broadcast message test data
Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/28317 and https://gitlab.com/gitlab-org/gitlab-ce/issues/29176