Document and deprecate a strange property.

This is a pretty odd property, that we should deprecate now to avoid
keeping it around much longer.

Refs #9894

src/Controller/Component/RequestHandlerComponent.php

@@ -91,6 +91,15 @@ class RequestHandlerComponent extends Component
         'enableBeforeRedirect' => true
     ];
 
+    /**
+     * Set the layout to be used when rendering the AuthComponent's ajaxLogin element.
+     *
+     * @var string
+     * @deprecated 3.3.11 This feature property is not supported and will
+     *   be removed in 4.0.0
+     */
+    public $ajaxLayout;
+
     /**
      * Constructor. Parses the accepted content types accepted by the client using HTTP_ACCEPT
      *