Fix a slew of mdoc warnings/errors.

hz.9: update stathz for current usage

Update the stathz description to reflect reality. profhz is the only
thing we should deprecate. Add some implementation notes that describe
the optimizations made to date.

Discusssed with: emaste
Reviewed by: kib (prior), jhb (prior), gbe
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D30815

man9: add hz(9) and hardclock(9)

Document aspects of system time keeping. Hz is the nominal rate that we
interrupt the system and is known and the 'tick' period of 1 / hz.
hardclock is the routine that does various bits of timekeeping. stathz
and profhz are documented as historical relics that are deprecated
and replaced by hwpmc.4 and others.

Reviewed by: phk@, mav@ and gnn@ (previous version)
Obtained from: hardclock.9 from NetBSD (with FreeBSD adjustments)
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D30802