minor #7827 [Security] Firewall context stateless tip (geoffrey-brier, javiereguiluz)

This PR was merged into the 2.7 branch.

Discussion
----------

[Security] Firewall context stateless tip

I recently faced this situation.
Only took me a few minutes to find out why but IMO it's worth adding a note ;)

Commits
-------

9533b68 Minor reword and reformatting
64f1fa0 Add security stateless tip

Author: xabbuh

reference/configuration/security.rst

@@ -517,6 +517,13 @@ multiple firewalls, the "context" could actually be shared:
             ),
         ));
 
+.. note::
+
+    The firewall context key is stored in session, so every firewall using it
+    must set its ``stateless`` option to ``false``. Otherwise, the context is
+    ignored and you won't be able to authenticate on multiple firewalls at the
+    same time.
+
 HTTP-Digest Authentication
 --------------------------