docs(admin): document HTTP 503 to simulate maintenance for clients

Signed-off-by: whoalin1 <whoalin414@gmail.com>

Author: whoalin1

admin_manual/maintenance/index.rst

@@ -13,3 +13,22 @@ Maintenance
    package_upgrade
    migrating
    migrating_owncloud
+
+
+Simulating maintenance mode for clients
+---------------------------------------
+
+When Nextcloud is fully stopped or taken offline for infrastructure work, it can
+no longer serve its normal status payload that tells desktop and mobile clients
+the instance is in maintenance mode.
+
+In that situation, configure the reverse proxy or load balancer to answer
+client-facing Nextcloud endpoints with **HTTP 503 Service Unavailable**. Clients
+treat a 503 the same way as Nextcloud's built-in maintenance mode and will back
+off instead of surfacing hard sync errors.
+
+Examples of endpoints clients hit include ``status.php``, ``index.php``, and
+WebDAV / OCS URLs under your Nextcloud base path.
+
+For application-level maintenance while PHP is still running, prefer
+``occ maintenance:mode --on`` (see :doc:`../configuration_server/config_sample_php_parameters`).