docs: add explanation

Author: kenjis

user_guide_src/source/incoming/incomingrequest/040.php

@@ -9,7 +9,7 @@
 $request->is('patch');
 $request->is('options');
 
-// Checks if it is an AJAX request.
+// Checks if it is an AJAX request. The same as `$request->isAJAX()`.
 $request->is('ajax');
 
 // Checks if it is a JSON request.