minor #6191 Document the invalidate_session option (javiereguiluz)

This PR was squashed before being merged into the 2.3 branch (closes #6191).

Discussion
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Document the invalidate_session option

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes
| Applies to    | all
| Fixed tickets | -

I found this missing doc while closing symfony/symfony#9543 and checking out the given reference to http://stackoverflow.com/questions/17129782/symfony2-logging-out-of-one-firewall

Commits
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291dc5a Document the invalidate_session option

Author: xabbuh

reference/configuration/security.rst

@@ -319,6 +319,22 @@ Redirecting after Login
 
 .. _reference-security-pbkdf2:
 
+Logout Configuration
+--------------------
+
+invalidate_session
+..................
+
+**type**: ``boolean`` **default**: ``true``
+
+By default, when users log out from any firewall, their sessions are invalidated.
+This means that logging out from one firewall automatically logs them out from
+all the other firewalls.
+
+The ``invalidate_session`` option allows to redefine this behavior. Set this
+option to ``false`` in every firewall and the user will only be logged out from
+the current firewall and not the other ones.
+
 Using the PBKDF2 Encoder: Security and Speed
 --------------------------------------------