minor #18215 Tweaks in the code examples of the controller early hints (javiereguiluz)

This PR was merged into the 6.3 branch.

Discussion
----------

Tweaks in the code examples of the controller early hints

Let's add a new and different `Link` example for a common need in web apps ... and let's tweak the code comment.

Commits
-------

813398d Tweaks in the code examples of the controller early hints

Author: javiereguiluz

controller.rst

@@ -542,11 +542,12 @@ method::
         public function index(): Response
         {
             $response = $this->sendEarlyHints([
+                new Link(rel: 'preconnect', href: 'https://fonts.google.com'),
                 (new Link(href: '/style.css'))->withAttribute('as', 'stylesheet'),
                 (new Link(href: '/script.js'))->withAttribute('as', 'script'),
             ]);
 
-            // Do something slow...
+            // prepare the contents of the response...
 
             return $this->render('homepage/index.html.twig', response: $response);
         }