Update explanation of job-level variable override to fit example
As requested by @grzesiek at
https://gitlab.com/gitlab-org/gitlab-ce/issues/30337#note_31513138
Do not update/delete: Banner broadcast message test data
Do not update/delete: Notification broadcast message test data
As requested by @grzesiek at
https://gitlab.com/gitlab-org/gitlab-ce/issues/30337#note_31513138