minor #17009 Added enable http cache xml example (mohamedGasmii)

This PR was merged into the 5.4 branch.

Discussion
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Added enable http cache xml example

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Commits
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bf85ab3 added enable http cache xml example

Author: javiereguiluz

http_cache.rst

@@ -89,6 +89,26 @@ Use the ``framework.http_cache`` option to enable the proxy for the
             framework:
                 http_cache: true
 
+    .. code-block:: xml
+
+        <!-- config/packages/framework.xml -->
+        <?xml version="1.0" encoding="UTF-8" ?>
+        <container xmlns="http://symfony.com/schema/dic/services"
+            xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+            xmlns:framework="http://symfony.com/schema/dic/symfony"
+            xsi:schemaLocation="http://symfony.com/schema/dic/services
+                https://symfony.com/schema/dic/services/services-1.0.xsd
+                http://symfony.com/schema/dic/symfony
+                https://symfony.com/schema/dic/symfony/symfony-1.0.xsd">
+
+            <when env="prod">
+              <framework:config>
+                  <!-- ... -->
+                  <framework:http-cache enabled="true"/>
+              </framework:config>
+            </when>
+        </container>
+
     .. code-block:: php
 
         // config/packages/framework.php