In the old grade book, there was an option to leave a comment directly on an assignment from the grade book. This is not available on the beta right now. This is a very valuable option that should be in the new grade book.
I agree @beatty . I'm assuming that it's just not ready yet but it will be there eventually.
Several colleagues and I have adopted the new gradebook, and this feature has been missed! We hope that it returns soon.
Comments are back in our new grade detail tray, as announced in Canvas Release: New Gradebook. You can also edit your own comments as well.
Hope that helps!