minor #10631 Document logout success_handler configuration (simshaun)

This PR was squashed before being merged into the 2.8 branch (closes #10631).

Discussion
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Document logout success_handler configuration

The [Security docs](https://symfony.com/doc/current/security.html#logging-out) tell how to control what happens after logging out, but the security configuration reference is missing documentation on the logout success_handler.

_Partially_ related to #4258

Commits
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b93452a Document logout success_handler configuration

Author: javiereguiluz

reference/configuration/security.rst

@@ -166,6 +166,14 @@ 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.
 
+success_handler
+~~~~~~~~~~~~~~~
+
+**type**: ``string`` **default**: ``'security.logout.success_handler'``
+
+The service ID used for handling a successful logout. The service must implement
+:class:`Symfony\\Component\\Security\\Http\\Logout\\LogoutSuccessHandlerInterface`.
+
 .. _reference-security-ldap:
 
 LDAP functionality