Under the "old" style Yahoo! Answers, I could add a comment once my answer was picked as best. Most of the time that's just to thank the person for picking my response as best, but sometimes to add additional information as well.
There is still a comment link in the "new" Y!A, but it does not work. I've tried it with IE (versions 8, 9, and 10) and the current releases of Chrome and FireFox so it does not appear to be browser specific. Oddly, I can add comments to other people's answers, but that's not what I want to do in most cases.
I've tried the Feedback link under the "gear" icon in the upper right, but that does not appear to work at all. There's a space to enter a heading, but no place to provide actual information or submit anything.
IMHO, the "new" Yahoo! Answers is not ready for prime time and is a major step backwards in usability.