Agreed, this exists on the Firewall configuration page and should be allowed there also. This is a good wish, and I know others who have wished for it. Please do make a wish and make it from that page in Dashboard, and wish for the comments box field in that 1:1 NAT section. Making that wish, with certain keywords, and from that page in Dashboard helps the wish analytics engine properly classify the wish for the appropriate back-end Dashboard development team. I'll check if there is a Feature Request in the systems and upvote it as well.