Incorporate Accessibility Addendum into Main Document

[Crissov]

The Chapter Frame Addendum, defining the ATXT frame, has been published on 20 November 2006, but has seen limited adoption, which might partially have been caused by it not being integrated into the main standard. This decision, however, did make some sense, because the addendum was intended to apply to versions 2.3 and 2.4 alike. Now it's finally time to incorporate it.