It would be great to be able to attach a document to a document comment rather than just posting text. You can in a discussion reply. Or is the idea to keep comments just short and simple?
Yes, your assumption is correct - the purpose of document comments are to be short and simple discussions about the document. More about feedback, reviewing, suggestions, etc. If you want to refer to another document, we suggest uploading that into Clearspace and then link to it from the comment.
The Jive Customers group here on Clearstep is a great place to ask these types of questions.