Explanation of Session/NamespacedAttributeBag usage

[alexwybraniec]

The docs for Sessions (setting up, configuring, extending) and especially NamespacedAttributeBag are pretty poor.

Seemingly this SO response gives the only clue as to how to make NamespacedAttributeBag work in Symfony 3.

http://stackoverflow.com/questions/12279247/how-to-use-namespaced-sessions-in-symfony2/12279483#12279483

I will attempt to improve the docs myself, but before I start is there a better reference to look at first?

[ThomasLandauer]

I've taken a first step to explain this: #7643

[javiereguiluz]

Fixed by #7643.