docs(admin): note that av_rescan_days=0 does not disable rescanning

Values below 1 are silently replaced with the 28-day default in
BackgroundScanner::getOutdatedFiles(), so setting 0 to switch off the periodic
rescan has no effect. Points at av_background_scan instead.

AI-Assisted-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Signed-off-by: Anna Larch <anna@nextcloud.com>

Author: miaulalala

admin_manual/configuration_server/antivirus_configuration.rst

@@ -231,3 +231,8 @@ Disabling background scan task
 You can disable background scan with occ to only scan files during upload::
 
     sudo -E -u www-data php occ config:app:set files_antivirus av_background_scan --value="off"
+
+.. note:: Setting ``av_rescan_days`` to ``0`` does **not** disable the periodic rescan of
+   already-scanned files. Any value below ``1`` is silently replaced with the default of 28 days,
+   so the rescan continues on its normal schedule. Use ``av_background_scan`` as shown above to
+   stop background scanning altogether.