Update service_closures.rst

Author: chx

service_container/service_closures.rst

@@ -49,13 +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
+.. versionadded:: 7.3
+
+                # A shorthand is available
+                # arguments: ['@>mailer']
+
+                # It also works when the dependency is optional
                 # arguments: ['@>?mailer']
 
     .. code-block:: xml