Explain how to use the NamespacedAttributeBag

There is nothing in the Session documentation explaining how to use the NamespacedAttributeBag for managing session.
And even in the case of the fullstack framework, it should not go to the parameters.yml file. This is a config related to the project itself, not to each local setup of the project. So it has no reason to be in the ignored file (it is actually a way to make it harder for your team)