Update documentation according to code

Author: mtarld

service_container/service_decoration.rst

@@ -382,38 +382,31 @@ Three different behaviors are available:
 * ``null``: When used, the container will keep the decorator
   service and will set the decorated one to `null`.
 
-.. note::
-
-    If you are using yaml to describe your services, please ensure that
-    ``null`` value is wrapped with quotes. In fact in yaml, ``null`` will be interpreted
-    as the null value and not as the null string. Therefore, the compiler will throw
-    an ``InvalidArgumentException``
-
-    .. code-block:: yaml
-
-        Foo:
-            decorates: Foo
-            decoration_on_invalid: 'null'
-            arguments: ['@Bar.inner']
-
 .. tip::
 
     When using ``null``, you may update the decorator's constructor in
     order to make decorated dependency nullable. In this way, you will be able to
     check the presence of decorated service in other methods.
 
+    .. code-block:: yaml
+
+        App\Service\DecoratorService:
+            decorates: Acme\OptionalBundle\Service\OptionalService
+            decoration_on_invalid: null
+            arguments: ['@App\Service\DecoratorService.inner']
+
     ::
 
         namespace App\Service;
 
-        use App\Service\OptionalDecoratedService;
+        use Acme\OptionalBundle\Service\OptionalService;
 
         class DecoratorService
         {
             private $decorated;
 
-            - public function __construct(OptionalDecoratedService $decorated)
-            + public function __construct(?OptionalDecoratedService $decorated)
+            - public function __construct(OptionalService $decorated)
+            + public function __construct(?OptionalService $decorated)
             {
                 $this->decorated = $decorated;
             }