mod_assign: Save changes does not save comments

We've had at least one know teacher who has provided valuable comment to students for nearly an entire semster all of which were lost because the "Save changes" button at the bottom only saves the grade not the comment.

From a UX standpoint, the "Save changes" button should either save comments (ideally, no reason this shouldn't happen) or it should be clearly indicated to the user that the Save changes button does not save comments.