minor #10495 Show how to change the Session Attribute Bag type (pbowyer)

This PR was submitted for the master branch but it was squashed and merged into the 4.2 branch instead (closes #10495).

Discussion
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Show how to change the Session Attribute Bag type

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?

Commits
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c145ed1 Show how to change the Session Attribute Bag type

Author: javiereguiluz

components/http_foundation/sessions.rst

@@ -207,6 +207,8 @@ Example::
     // ...
     $session->clear();
 
+.. _namespaced-attributes:
+
 Namespaced Attributes
 .....................
 

session.rst

@@ -201,5 +201,6 @@ More about Sessions
     session/locale_sticky_session
     session/php_bridge
     session/proxy_examples
+    session/attribute_bag
 
 .. _`HttpFoundation component`: https://symfony.com/components/HttpFoundation

session/attribute_bag.rst

@@ -0,0 +1,33 @@
+.. index::
+   single: Sessions, Attribute Bag
+
+Changing the Session Attribute Bag Provider
+===========================================
+
+Symfony uses attribute bags to represent values stored in the session. It ships 
+with the default :class:`Symfony\\Component\\HttpFoundation\\Session\\Attribute\\AttributeBag`, 
+which stores key-value pairs, and the 
+:class:`Symfony\\Component\\HttpFoundation\\Session\\Attribute\\NamespacedAttributeBag`, 
+which allows you to :ref:`set and fetch values using character-separated paths <namespaced-attributes>`.
+
+This is fully documented :doc:`in the component documentation </components/http_foundation/sessions>`.
+
+Changing the Attribute Bag in Symfony
+-------------------------------------
+
+To use a different attribute bag for your sessions, override the service 
+definition. In this example, we switch the default ``AttributeBag`` to the 
+``NamespacedAttributeBag``:
+
+.. configuration-block::
+
+    .. code-block:: yaml
+
+        # config/services.yaml
+        session:
+            class: Symfony\Component\HttpFoundation\Session\Session
+            arguments: ["@session.storage", "@session.namespacedattributebag", "@session.flash_bag"]
+
+        session.namespacedattributebag:
+            class: Symfony\Component\HttpFoundation\Session\Attribute\NamespacedAttributeBag
+