docs(nextcloud): update security warning decisions and background job annotation

update notes:
- trusted_proxies warning
- HSTS option warning
- background job mode annotation

config/services/containers/app/nextcloud/config/background.config.php.j2

@@ -1,4 +1,5 @@
 <?php
 $CONFIG = [
+    // Background jobs mode is auto-detected as 'cron' when nextcloud-cron.timer runs cron.php via CLI. No explicit config required.
     'maintenance_window_start' => 18,
 ];