Remove doc for debugging callbacks. Methods don't exist in Rails master

activerecord/lib/active_record/callbacks.rb

@@ -218,16 +218,6 @@ module ActiveRecord
   # needs to be aware of it because an ordinary +save+ will raise such exception
   # instead of quietly returning +false+.
   #
-  # == Debugging callbacks
-  #
-  # To list the methods and procs registered with a particular callback, append <tt>_callback_chain</tt> to
-  # the callback name that you wish to list and send that to your class from the Rails console:
-  #
-  #   >> Topic.after_save_callback_chain
-  #   => [#<ActiveSupport::Callbacks::Callback:0x3f6a448
-  #       @method=#<Proc:0x03f9a42c@/Users/foo/bar/app/models/topic.rb:43>, kind:after_save, identifiernil,
-  #       options{}]
-  #
   module Callbacks
     extend ActiveSupport::Concern