diff --git a/core/modules/user/user.admin.inc b/core/modules/user/user.admin.inc
index 06ac8d2..8efc510 100644
--- a/core/modules/user/user.admin.inc
+++ b/core/modules/user/user.admin.inc
@@ -5,6 +5,13 @@
  * Admin page callback file for the user module.
  */
 
+/**
+ * Page callback: Generates the appropriate user administration form.
+ *
+ * @param string $callback_arg
+ *   (optional) Indicates which form to build. Defaults to '', which will
+ *   trigger the user filter form.
+ */
 function user_admin($callback_arg = '') {
   $op = isset($_POST['op']) ? $_POST['op'] : $callback_arg;