Also would like this, it would save a lot of time when trying to recall why a change was made. e.g. Why did I switch this job to run against version x instead of y?
Preferred UI would be a "reason" box at the end of the config (above save/apply) and then ability to add/modify from the job configuration history page.
Probably unrelated, but can the "Diff" column be moved to the far left or date to the right? When trying to find a change its painful to have to count down each time rather than being able to associate the previously selected diff with the date on the left.
So what would be the preferred UI for this? A textbox at the end of the job/jenkins configuration pages?