Do not update/delete: Banner broadcast message test data
Do not update/delete: Notification broadcast message test data
closes https://gitlab.com/gitlab-com/www-gitlab-com/issues/1393
Documents https://gitlab.com/gitlab-com/www-gitlab-com/merge_requests/5937, added by https://gitlab.com/gitlab-com/www-gitlab-com/merge_requests/5551
cc/ @SeanPackham @axil @JobV