mdoc.7/MANUAL STRUCTURE: add HARDWARE

FreeBSD Release infrastructure has been generating the Hardware
Compatibility Notes from this section for some decades. Make this more
obvious. This does change the upstream mdoc.7 in a minimal way to
document this, but the .Dd wasn't bumped to avoid conflicts with the
next mdoc/mandoc import.

Reported by: bz (HARDWAREs generating compatibility notes)
Reported by: grahamperrin (order listed in fdp-primer/man)
Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/1463