Mark ActionFilter as nodoc

Didn't intend for this to be public. Oops!

Author: jhawthorn

actionpack/lib/abstract_controller/callbacks.rb

@@ -35,7 +35,7 @@ module Callbacks
                        skip_after_callbacks_if_terminated: true
     end
 
-    class ActionFilter
+    class ActionFilter # :nodoc:
       def initialize(actions)
         @actions = Array(actions).map(&:to_s).to_set
       end