minor #11263 Add an example to the invokable controllers section (javiereguiluz)

This PR was squashed before being merged into the 3.4 branch (closes #11263).

Discussion
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Add an example to the invokable controllers section

Fixes #7549. Invokable controllers were already explained, but we can expand the explanation a bit with an actual example.

Commits
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6259e80 Add an example to the invokable controllers section

Author: OskarStark

controller/service.rst

@@ -78,9 +78,24 @@ syntax:
 Invokable Controllers
 ---------------------
 
-If your controller implements the ``__invoke()`` method - popular with the
-Action-Domain-Response (ADR) pattern, you can simply refer to the service id
-(``AppBundle\Controller\HelloController`` or ``app.hello_controller`` for example).
+Controllers can also define a single action using the ``__invoke()`` method,
+which is a common practice when following the `ADR pattern`_
+(Action-Domain-Responder)::
+
+    // src/AppBundle/Controller/Hello.php
+    use Symfony\Component\HttpFoundation\Response;
+    use Symfony\Component\Routing\Annotation\Route;
+
+    /**
+     * @Route("/hello/{name}", name="hello")
+     */
+    class Hello
+    {
+        public function __invoke($name = 'World')
+        {
+            return new Response(sprintf('Hello %s!', $name));
+        }
+    }
 
 Alternatives to base Controller Methods
 ---------------------------------------
@@ -139,3 +154,4 @@ If you want to know what type-hints to use for each service, see the
 .. _`base Controller class`: https://github.com/symfony/symfony/blob/master/src/Symfony/Bundle/FrameworkBundle/Controller/ControllerTrait.php
 .. _`ControllerTrait`: https://github.com/symfony/symfony/blob/master/src/Symfony/Bundle/FrameworkBundle/Controller/ControllerTrait.php
 .. _`AbstractController`: https://github.com/symfony/symfony/blob/master/src/Symfony/Bundle/FrameworkBundle/Controller/AbstractController.php
+.. _`ADR pattern`: https://en.wikipedia.org/wiki/Action%E2%80%93domain%E2%80%93responder