Explain how to use the NamespacedAttributeBag

There is nothing in the Session documentation explaining how to use the NamespacedAttributeBag for managing session.
The trick of adding `::` at the end of a paragraph can only be used when the following code is PHP. In this case we have YAML, so you should use:

```
[...] your ``parameters.yml`` file should contain:

.. code-block:: yaml

    parameters:
        session.attribute_bag.class: Symfony\Component\HttpFoundation\Session\Attribute\NamespacedAttributeBag
```