minor #5943 Add tip for when returning null from createToken() (jeroenseegers)

This PR was submitted for the 2.8 branch but it was merged into the 2.7 branch instead (closes #5943).

Discussion
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Add tip for when returning `null` from `createToken()`

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | #5264

Commits
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1337b31 Add caution when returning null from createToken()

Author: xabbuh

cookbook/security/api_key_authentication.rst

@@ -109,6 +109,11 @@ will cause authentication to fail. You might want to return ``null`` instead
 to just skip the authentication, so Symfony can fallback to another authentication
 method, if any.
 
+.. caution::
+
+    In case you return ``null`` from your ``createToken()`` method, be sure to enable
+    ``anonymous`` in you firewall. This way you'll be able to get an ``AnonymousToken``.
+
 2. supportsToken
 ~~~~~~~~~~~~~~~~