minor #9241 Improved a routing example to show annotations (javiereguiluz)

This PR was squashed before being merged into the 2.7 branch (closes #9241).

Discussion
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Improved a routing example to show annotations

This fixes the first issue of #9159.

Commits
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d553d70 Improved a routing example to show annotations

Author: javiereguiluz

routing/external_resources.rst

@@ -92,15 +92,29 @@ in that directory are parsed and put into the routing.
 
             return $collection;
 
-Prefixing Imported Routes
-~~~~~~~~~~~~~~~~~~~~~~~~~
+.. _prefixing-imported-routes:
+
+Prefixing the URLs of Imported Routes
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 You can also choose to provide a "prefix" for the imported routes. For example,
 suppose you want to prefix all routes in the AppBundle with ``/site`` (e.g.
 ``/site/blog/{slug}`` instead of ``/blog/{slug}``):
 
 .. configuration-block::
 
+    .. code-block:: php-annotations
+
+        use Sensio\Bundle\FrameworkExtraBundle\Configuration\Route;
+
+        /**
+         * @Route("/site")
+         */
+        class DefaultController
+        {
+            // ...
+        }
+
     .. code-block:: yaml
 
         # app/config/routing.yml