minor #4908 Set twig service as private (ifdattic)

This PR was merged into the 2.3 branch.

Discussion
----------

Set twig service as private

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.3
| Fixed tickets |

Commits
-------

b71b007 Set twig service as private

cookbook/templating/twig_extension.rst

@@ -74,14 +74,17 @@ Now you must let the Service Container know about your newly created Twig Extens
         services:
             app.twig_extension:
                 class: AppBundle\Twig\AppExtension
+                public: false
                 tags:
                     - { name: twig.extension }
 
     .. code-block:: xml
 
         <!-- app/config/services.xml -->
         <services>
-            <service id="app.twig_extension" class="AppBundle\Twig\AppExtension">
+            <service id="app.twig_extension"
+                class="AppBundle\Twig\AppExtension"
+                public="false">
                 <tag name="twig.extension" />
             </service>
         </services>
@@ -93,6 +96,7 @@ Now you must let the Service Container know about your newly created Twig Extens
 
         $container
             ->register('app.twig_extension', '\AppBundle\Twig\AppExtension')
+            ->setPublic(false)
             ->addTag('twig.extension');
 
 .. note::