Add note about PHP 5.3 behaviour and proxies.

Author: Drak

components/http_foundation/session_configuration.rst

@@ -252,12 +252,18 @@ using the `Native*SessionHandler` classes, while
 :class:`Symfony\\Component\\HttpFoundation\\Session\\Storage\\Handler\\SessionHandlerProxy`
 will be used to wrap any custom save handlers, that implement :phpclass:`SessionHandlerInterface`.
 
-Under PHP 5.4 and above, all session handlers implement :phpclass:`SessionHandlerInterface`
+From PHP 5.4 and above, all session handlers implement :phpclass:`SessionHandlerInterface`
 including `Native*SessionHandler` classes which inherit from :phpclass:`SessionHandler`.
 
 The proxy mechanism allows you to get more deeply involved in session save handler
 classes. A proxy for example could be used to encrypt any session transaction
 without knowledge of the specific save handler.
 
+.. note::
+
+    Under PHP 5.3, you cannot proxy the session handler interface for native PHP handlers, 
+    only user handlers so if you write proxies, be aware they will only work for user-land 
+    session save handlers.
+
 .. _`php.net/session.customhandler`: http://php.net/session.customhandler
 .. _`php.net/session.configuration`: http://php.net/session.configuration