Issue #1037352 by jhodgdon: hook_permission() is missing doc about 'warning'.

modules/system/system.api.php

@@ -1911,6 +1911,9 @@ function hook_system_info_alter(&$info, $file, $type) {
  *     warning message defined in user_admin_permissions() will be associated
  *     with the permission and displayed with it on the permission
  *     administration page. Defaults to FALSE.
+ *   - warning: (optional) A translated warning message to display on the
+ *     permission administration page. This warning overrides the automatic
+ *     warning generated by 'restrict access' being set to TRUE.
  */
 function hook_permission() {
   return array(