minor #18050 Be consistent in code examples (OskarStark)

This PR was squashed before being merged into the 5.4 branch.

Discussion
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Be consistent in code examples

Follows #15731 (comment)

cc `@HeahDude`

Commits
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2426ce4 Be consistent in code examples

Author: OskarStark

service_container/service_closures.rst

@@ -55,6 +55,9 @@ 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 -->
@@ -66,6 +69,11 @@ 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>