James K. Lowden wrote:

> Problem is, unless I'm blind, is that I've got mdoc(7) and man(7), but
> precious little on troff, whose page says, "Only the features not in Unix
> troff are described here", and of course /usr/share/doc/usd/21.troff/ is
> empty.  I also didn't find any troff docs at the Unix Historical Society. 

Bell Labs is your friend. Googling for "Troff User's Manual", you'll get
as the first link:

http://cm.bell-labs.com/sys/doc/troff.pdf

-- 
arto