Google has improved the commenting system available in docs. You can now carry on a discussion with collaborators while working on a document. The feature should be available now, but you will not see it (red box in image shows the way to access the feature) unless you create a new document.
Google provides a video with additional information.
This would seem an effective way to leave comments an author might either implement by changing the document or respond to as part of what Google would like to call a discussion.
