Add nodoc to a private constant [ci skip]

Author: rafaelfranca

actionpack/lib/action_dispatch/middleware/cookies.rb

@@ -181,7 +181,7 @@ def signed_or_encrypted
     # to the Message{Encryptor,Verifier} allows us to handle the
     # (de)serialization step within the cookie jar, which gives us the
     # opportunity to detect and migrate legacy cookies.
-    module VerifyAndUpgradeLegacySignedMessage
+    module VerifyAndUpgradeLegacySignedMessage # :nodoc:
       def initialize(*args)
         super
         @legacy_verifier = ActiveSupport::MessageVerifier.new(@options[:secret_token], serializer: ActiveSupport::MessageEncryptor::NullSerializer)