Update the documentation to say that we require the virtual attribute to be…

Update the documentation to say that we require the virtual attribute to be defined in order to use the validation.

lib/paperclip.rb

@@ -351,6 +351,8 @@ module Paperclip
     # NOTE: If you do not specify an [attachment]_content_type field on your
     # model, content_type validation will work _ONLY upon assignment_ and
     # re-validation after the instance has been reloaded will always succeed.
+    # You'll still need to have a virtual attribute (created by +attr_accessor+)
+    # name +[attachment]_content_type+ to be able to use this validator.
     def validates_attachment_content_type name, options = {}
       validation_options = options.dup
       allowed_types = [validation_options[:content_type]].flatten