Adding default PHP format

Info is taken from #17515 (comment)

Author: ThomasLandauer

routing.rst

@@ -151,8 +151,8 @@ the ``BlogController``:
 
 .. note::
 
-    By default Symfony only loads the routes defined in YAML format. If you
-    define routes in XML and/or PHP formats, you need to
+    By default Symfony only loads the routes defined in YAML and PHP format.
+    If you define routes in XML, you need to
     :ref:`update the ``src/Kernel.php`` file <configuration-formats>`.
 
 .. _routing-matching-http-methods: