Document broadcast messages in UI

What does this MR do?

Related issues

Relates to: https://gitlab.com/gitlab-org/gitlab-ce/issues/45801.

Author's checklist

• Apply the correct labels and milestone
• Crosslink the document from the higher-level index
• Port the MR to EE (or backport from CE): https://gitlab.com/gitlab-org/gitlab-ce/issues/45801

Review checklist

• PM's review (recommended, but not a blocker)
• Technical writer's review (required)
• Merge the EE-MR first, CE-MR (https://gitlab.com/gitlab-org/gitlab-ce/issues/45801) afterwards