Document broadcast messages in UI

What does this MR do?

The broadcast messages UI has never been documented.

This fixes that.

Related issues

Closes 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-ee/merge_requests/9248.

Review checklist

• PM's review (recommended, but not a blocker)
• Technical writer's review (required)
• Merge the EE-MR first (https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9248), CE-MR afterwards