mdoc(7): improve output of .At 32v

mdoc(7): improve output of .At 32v

The official designation by AT&T was "UNIX/32V", so use that in the output.
That also makes sense because "system/architecture" is a widespread
convention to refer to the port of an operating system to a specific
architecture, in this case 32V (32bit DEC VAX).

The former wording "Version 32V AT&T UNIX" was misleading
because 32V is not a version number.

Even though UNIX/32V was not officially designated as Version 7 by AT&T,
prepend "Version 7" because it was in fact a straightforward port of
Version 7 AT&T UNIX.  That makes it easier to understand for 21st
century readers of manual pages.

Suggested by nabijaczleweli at nabijaczleweli dot xyz.
Same change as in GNU troff commit 21d30728.
OK  G dot Branden dot Robinson at gmail dot com  (gbranden@ in groff)