minor #14861 Listing possible values of storage_id (ThomasLandauer)

This PR was squashed before being merged into the 4.4 branch.

Discussion
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Listing possible values of storage_id

Taken from https://stackoverflow.com/a/29563414/1668200

I think there should be a *full* list of possible values. And the existence of this SO question shows that there is none ;-)

Commits
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3d560c3 Listing possible values of storage_id

Author: javiereguiluz

reference/configuration/framework.rst

@@ -1249,9 +1249,14 @@ storage_id
 
 **type**: ``string`` **default**: ``'session.storage.native'``
 
-The service id used for session storage. The ``session.storage`` service
-alias will be set to this service id. This class has to implement
+The service ID used for storing the session. The ``session.storage`` service
+alias will be set to this service. The class has to implement
 :class:`Symfony\\Component\\HttpFoundation\\Session\\Storage\\SessionStorageInterface`.
+To see a list of all available storages, run:
+
+.. code-block:: terminal
+
+    $ php bin/console debug:container session.storage.
 
 .. _config-framework-session-handler-id: