Import changes by schwarze@ and Jason McIntyre into manuals. These are

Import changes by schwarze@ and Jason McIntyre into manuals.  These are
for the new libroff macros, the new libman macros, and for the sake of
general clarity.

Note that `ft' documentation is NOT included, as I've kept it in -man:
it's highly confusing to have it in roff.7 when it's not implemented for
both -man and -mdoc, as at least in theory, roff.7 should include only
macros recognised by BOTH formats.