Add more precision about automatic provider assignation

Author: AbdelKader Bouadjadja

security/multiple_user_providers.rst

@@ -82,7 +82,7 @@ a new provider that chains the two together:
             ),
         ));
 
-Now, all authentication mechanisms will use the ``chain_provider``, since
+Now, all firewalls without explicit provider configured will use the ``chain_provider``, since
 it's the first specified. The ``chain_provider`` will, in turn, try to load
 the user from both the ``in_memory`` and ``user_db`` providers.