Improve TouchBarButton accessibility

Editor assessment

The verb improve is doing nearly all the work in this subject: TouchBarButton accessibility got better, but nothing says how, whether new labels, traits, or announcements were added, or what was deficient before. The feat type is plausible for an accessibility enhancement. No scope identifies the TouchBar area in form, no body exists to enumerate the improvements, and no footers appear. Hygiene is otherwise clean. A reader interested in accessibility cannot learn anything actionable from the message and must read the diff, which is the weakness the subject-clarity dimension exists to catch.