Minor session documentation change.

doc/providers/session.rst

@@ -83,3 +83,17 @@ authenticates a user and creates a session for him::
 
         return "Welcome {$user['username']}!";
     });
+
+
+Custom Session Configurations
+-----------------------------
+
+If your system is using a custom session configuration (such as a redis handler
+from a PHP extension) then you need to disable the NativeFileSessionHandler by
+setting ``session.storage.handler`` to null. You will have to configure the
+``session.save_path`` ini setting yourself in that case.
+
+.. code-block:: php
+
+    $app['session.storage.handler'] = null;
+