add link to MongoDB cookbook article

Author: stevenmusumeche

cookbook/session/sessions_directory.rst

@@ -93,8 +93,9 @@ that your current sessions aren't lost when you clear Symfony's cache.
     Using a different session save handler is an excellent (yet more complex)
     method of session management available within Symfony. See
     :doc:`/components/http_foundation/session_configuration` for a
-    discussion of session save handlers. There is also an entry in the cookbook
-    about storing sessions in the :doc:`database </cookbook/configuration/pdo_session_storage>`.
+    discussion of session save handlers. There are also entries in the cookbook
+    about storing sessions in a :doc:`relational database </cookbook/configuration/pdo_session_storage>`
+    or a :doc:`NoSQL database </cookbook/configuration/mongodb_session_storage>`.
 
 To change the directory in which Symfony saves session data, you only need
 change the framework configuration. In this example, you will change the