Add small fixes in Canary deployments docs
What does this MR do?
Add post-improvements from https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/2359 discussion.
Do not update/delete: Banner broadcast message test data
Do not update/delete: Notification broadcast message test data
Add post-improvements from https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/2359 discussion.