Documented the new @> shorthand

Author: chx

service_container/service_closures.rst

@@ -49,9 +49,15 @@ argument of type ``service_closure``:
             App\Service\MyService:
                 arguments: [!service_closure '@mailer']
 
+                # A shorthand is available
+                # arguments: ['@>mailer']
+
                 # In case the dependency is optional
                 # arguments: [!service_closure '@?mailer']
 
+                # Using the shorthand
+                # arguments: ['@>?mailer']
+
     .. code-block:: xml
 
         <!-- config/services.xml -->