Be consistent in code examples

Author: OskarStark

service_container/service_closures.rst

@@ -58,6 +58,9 @@ argument of type ``service_closure``:
             App\Service\MyService:
                 arguments: [!service_closure '@mailer']
 
+                # In case the dependency is optional
+                # arguments: [!service_closure '@?mailer']
+
     .. code-block:: xml
 
         <!-- config/services.xml -->
@@ -69,6 +72,11 @@ argument of type ``service_closure``:
             <services>
                 <service id="App\Service\MyService">
                     <argument type="service_closure" id="mailer"/>
+
+                    <!--
+                    In case the dependency is optional
+                    <argument type="service_closure" id="mailer" on-invalid="ignore"/>
+                    -->
                 </service>
             </services>
         </container>