minor #8125 Add information for enable property_info service (Kleinast, javiereguiluz)

This PR was submitted for the 3.3 branch but it was merged into the 2.8 branch instead (closes #8125).

Discussion
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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.

Commits
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ffe0dc0 Reworded the tip about property_info and Symfony framework
77f6a6d remove useless space
01563dd Add information for enable property_info service

Author: xabbuh

components/property_info.rst

@@ -351,8 +351,17 @@ Using PHP reflection, the :class:`Symfony\\Component\\PropertyInfo\\Extractor\\R
 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.
+.. note::
+
+    When using the Symfony framework, this service is automatically registered
+    when the ``property_info`` feature is enabled:
+
+    .. code-block:: yaml
+
+        # app/config/config.yml
+        framework:
+            property_info:
+                enabled: true
 
 .. code-block:: php