Show how to change the Session Attribute Bag type

[pbowyer]

The Session component ships with two different attribute bag implementations, and the Symfony docs don't show how to change between them.

Fixes #10494 and StackOverflow issues like https://stackoverflow.com/questions/12279247/how-to-use-namespaced-sessions-in-symfony2

Specific feedback requests

1. Is this the right location for it? Adding it to https://github.com/symfony/symfony-docs/blob/master/session.rst was my first idea, but everything else is sub-pages.
2. I only use YAML configuration, so if someone else can contribute/check the PHP/XML config that would be brilliant.
3. Hard line wrapping - you appear to use ~80 characters? I've got some lines that go over that but don't think paths to files can be split?

[xabbuh]

I think we should not replace the session service, but only replace session.attribute_bag instead.

[OskarStark]

Please add a new line

[pbowyer]

Below line 31?

[javiereguiluz]

Peter, thanks for contributing these docs. We merged it in 4.2 branch (and then, master too) and we reworded it a bit. Instead of creating a short separate article ... we put this new info inside the main sessions article. Cheers!