minor #11587 Added some missing info in the article about container params in routes (javiereguiluz)

This PR was merged into the 3.4 branch.

Discussion
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Added some missing info in the article about container params in routes

Fixes #11581.

Commits
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8fac2e6 Added some missing info in the article about container params in routes

Author: OskarStark

routing/service_container_parameters.rst

@@ -164,6 +164,36 @@ path):
 
         return $routes;
 
+Now make sure that the ``app.route_prefix`` parameter is set somewhere in your
+container:
+
+.. configuration-block::
+
+    .. code-block:: yaml
+
+        # app/config/config.yml
+        parameters:
+            app.route_prefix: 'foo'
+
+    .. code-block:: xml
+
+        <!-- app/config/config.xml -->
+        <?xml version="1.0" charset="UTF-8" ?>
+        <container xmlns="http://symfony.com/schema/dic/services"
+            xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+            xsi:schemaLocation="http://symfony.com/schema/dic/services
+                https://symfony.com/schema/dic/services/services-1.0.xsd">
+
+            <parameters>
+                <parameter key="app.route_prefix">foo</parameter>
+            </parameters>
+        </container>
+
+    .. code-block:: php
+
+        // app/config/config.php
+        $container->setParameter('app.route_prefix', 'foo');
+
 .. note::
 
     Just like in normal service container configuration files, if you actually