chore: add docs for false_quick_deny

etc/emqx.conf

@@ -648,7 +648,10 @@ log.rotation.count = 5
 ## Allow anonymous authentication by default if no auth plugins loaded.
 ## Notice: Disable the option in production deployment!
 ##
-## Value: true | false
+## Configured to `false_quick_deny` means quick deny of anonymous clients
+## (without username). So the auth backend checks can be skipped.
+##
+## Value: true | false | false_quick_deny
 allow_anonymous = true
 
 ## Allow or deny if no ACL rules matched.