Added nortes as per @stof's recommendation.

Author: Drak

components/http_foundation/session_php_legacy.rst

@@ -9,15 +9,15 @@ Sometimes it may be necessary to integrate Symfony into a legacy application
 where you do not initially have the level of control you require.
 
 As stated elsewhere, Symfony Sessions are designed to replace the use of
-PHP's native `session_*()` functions and use of the `$_SESSION`
+PHP's native ``session_*()`` functions and use of the ``$_SESSION``
 superglobal. Additionally, it is mandatory for Symfony to start the session.
 
 However when there really are circumstances where this is not possible, it is possible
 to use a special storage bridge
 :class:`Symfony\\Component\\HttpFoundation\\Session\\Storage\\PhpSessionStorage`
 which is designed to allow Symfony to work with a session started outside of
 the Symfony Session framework. You are warned that things can interrupt this
-use case unless you are careful: for example legacy application erases `$_SESSION`.
+use case unless you are careful: for example legacy application erases ``$_SESSION``.
 
 Typical use of this might look as follows::
 
@@ -37,4 +37,13 @@ Typical use of this might look as follows::
     $session->start();
 
 This will allow you to start using the Symfony Session API and allow
-migration of your application to Symfony Sessions.
+migration of your application to Symfony Sessions.
+
+.. note::
+
+Symfony Sessions store data like attributes in special 'Bags' which use a
+key in the ``$_SESSION`` superglobal. This means that a Symfony Session
+cannot access arbitary keys in ``$_SESSION`` that may be set by the legacy
+application, although all the ``$_SESSION`` contents will be saved when
+the session is saved.
+