Add information for enable property_info service

add code for enable property_info service because service is not available by default in symfony full stack.
Also, I don't really understand why the property_info is mentionned in ReflectionExtractor section because property_info service lead to Symfony\Component\PropertyInfo\PropertyInfoExtractor and not the ReflectionExtractor class.

Author: Kleinast

components/property_info.rst

@@ -352,7 +352,14 @@ provides list, type and access information from setter and accessor methods.
 It can also provide return and scalar types for PHP 7+.
 
 This service is automatically registered with the ``property_info`` service in
-the Symfony Framework.
+the Symfony Framework, you just have to enable it in config.yml.
+
+.. code-block:: yaml
+
+    framework:
+        property_info:
+            enabled: true
+
 
 .. code-block:: php