Docs: Add note regarding topic visibility to admin area page
What does this MR do and why?
This MR adds a note regarding visibility of topics to the documentation of the admin area in doc/user/admin_area/index.md (according to suggestion in !72299 (comment 726308281)).
MR acceptance checklist
This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.
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