docs(admin): warn about OPcache delay after enabling maintenance mode - #15716

Summary

Clarify on the backup page that enabling maintenance mode is not always immediately effective for web workers because of PHP OPcache.

• Warn that FPM / mod_php workers may keep the previous config.php until opcache.revalidate_freq elapses (default often 2; some installs use 60).
• Tell scripted backups to wait at least that long, or reload PHP-FPM / mod_php, before dumping the DB or copying data.
• Note that occ maintenance:mode status alone does not prove web workers have revalidated.
• Cross-link server tuning (Enable PHP OPcache).

Closes #9184

Test plan

• Sphinx build of the admin manual succeeds for backup.rst
• New .. warning:: appears at the end of Maintenance mode, before Backup folders
• Mentions opcache.revalidate_freq, wait-or-reload, and that CLI status is insufficient
• Link to server_tuning resolves
• No changes to migrating.rst / server_tuning.rst

AI disclosure

This PR was drafted with the help of AI coding assistants (Cursor agents / LLM-based tools), including the description. I am responsible for it and happy to rework anything that doesn't fit.

All commits carry an Assisted-by: Cursor:grok-4.7 trailer.