Update docs for requesthandler replacement.

I'll backport this to 4.x so that the 5.x migration guide docs provide
the solution discussed as part of cakephp/cakephp#17415

Author: markstory

en/controllers.rst

@@ -374,6 +374,22 @@ view logic::
 It is important to remember that match-all views are applied only *after*
 content-type negotiation is attempted.
 
+Using AjaxView
+==============
+
+In applications that use hypermedia or AJAX clients, you often need to render
+view contents without the wrapping layout. You can use the ``AjaxView`` that
+is bundled with the application skeleton::
+
+    // In a controller action, or in beforeRender.
+    if ($this->request->is('ajax')) {
+        $this->viewBuilder()->setClassName('Ajax');
+    }
+
+``AjaxView`` will respond as ``text/html`` and use the ``ajax`` layout.
+Generally this layout is minimal or contains client specific markup. This
+replaces usage of ``RequestHandlerComponent`` automatically using the
+``AjaxView`` in 4.x.
 
 Redirecting to Other Pages
 ==========================

en/views.rst

@@ -117,9 +117,9 @@ Another example, using if/elseif/else. Notice the colons:
      <h3>Hi unknown user</h3>
   <?php endif; ?>
 
-If you'd prefer using a templating language like
-`Twig <https://twig.symfony.com>`_, a subclass of View will bridge your
-templating language and CakePHP.
+If you'd prefer to use a templating language like
+`Twig <https://twig.symfony.com>`_, checkout the `CakePHP Twig Plugin
+<https://github.com/cakephp/twig-view>`__ 
 
 Template files are stored in **templates/**, in a folder named after the
 controller that uses the files, and named after the action it corresponds to.