Document highlighted mentions

What does this MR do?

Document https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/21406

Related issues

https://gitlab.com/gitlab-org/gitlab-ce/issues/49036

Author's checklist

• Apply the correct labels and milestone
• Port the MR to EE (or backport from CE): always recommended, required when the ee-compat-check job fails

Review checklist

• Technical writer's review (required)
• Merge the EE-MR first, CE-MR afterwards