diff --git a/core/modules/system/system.routing.yml b/core/modules/system/system.routing.yml
index 9ef3720..2da5126 100644
--- a/core/modules/system/system.routing.yml
+++ b/core/modules/system/system.routing.yml
@@ -448,6 +448,8 @@ system.batch_page.json:
   options:
     _admin_route: TRUE
 
+# Note: This route just exists for generating URLs, the dedicated
+# frontcontroller is used if the URL is accessed.
 system.db_update:
   path: '/update.php/{op}'
   defaults: