My suggestion will be to make a WordPress Blog and write your Documentation there. Then you will be just updating that post. And in the .zip that you send to envato you will be sending .pdf of that blog post.
Here is mine http://www.nextwebart.com/support/restaurant_app_template_docs/ and I find this way very easy and useful.
I think it will be great feature specially for codecanyon authors when they update the item, to have special form to write about the changes they made, migration instructions etc. And this info to come directly to the emails of the users who have purchased the item.
What do you think?