minor #14551 [Security] Documented the NullToken usage (wouterj)

This PR was merged into the 5.2 branch.

Discussion
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[Security] Documented the NullToken usage

Fixes #14011

Commits
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3a439bd [#14011] Documented the NullToken usage

Author: javiereguiluz

security/experimental_authenticators.rst

@@ -135,6 +135,42 @@ unauthenticated access (e.g. the login page):
             ],
         ]);
 
+Granting Anonymous Users Access in a Custom Voter
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. versionadded:: 5.2
+
+    The ``NullToken`` class was introduced in Symfony 5.2.
+
+If you're using a :doc:`custom voter </security/voters>`, you can allow
+anonymous users access by checking for a special
+:class:`Symfony\\Component\\Security\\Core\\Authentication\\Token\\NullToken`. This token is used
+in the voters to represent the unauthenticated access::
+
+    // src/Security/PostVoter.php
+    namespace App\Security;
+
+    // ...
+    use Symfony\Component\Security\Core\Authentication\Token\NullToken;
+    use Symfony\Component\Security\Core\Authentication\Token\TokenInterface;
+    use Symfony\Component\Security\Core\Authorization\Voter\Voter;
+
+    class PostVoter extends Voter
+    {
+        // ...
+
+        protected function voteOnAttribute(string $attribute, $subject, TokenInterface $token): bool
+        {
+            // ...
+
+            if ($token instanceof NullToken) {
+                // the user is not authenticated, e.g. only allow them to
+                // see public posts
+                return $subject->isPublic();
+            }
+        }
+    }
+
 .. _authenticators-required-entry-point:
 
 Configuring the Authentication Entry Point