Mention that .AT, .P, .SB, and .UC are extensions; it really matters
because we only retain the language for backward compatibility in
the first place. Part of the research done by <G dot Branden dot
Robinson at gmail dot com>, see the list <groff at gnu dot org> for
details.
No change to the following conventions: Consider portable whatever
made it into GNU troff no later than 4.4BSD. For portable extensions,
mention their origin at the end of the description. For non-portable
extensions, for example from man-ext, usually warn earlier, near
the beginning of the description.