FrameworkBundle: Client: getContainer(): fixed phpdoc

The kernel might be shut down and then the method will return null instead of a ContainerInterface object

src/Symfony/Bundle/FrameworkBundle/Client.php

@@ -41,7 +41,7 @@ class Client extends BaseClient
     /**
      * Returns the container.
      *
-     * @return ContainerInterface
+     * @return ContainerInterface|null Returns null when the Kernel has been shutdown or not started yet
      */
     public function getContainer()
     {