hardclock(9): small cleanups

- Improve the wording that describes cnt and hz (avoid "the long haul")
- Use .Fa for function arguments
- Fix formatting of parenthesis around swi(9) xref
- removed xref to non-existent options(4)
- xref polling(4)
- consistently list xrefs in SEE ALSO
- xref eventtimers(9) in SEE ALSO

Reviewed by: mav, rpokala
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D39137

hardclock.9: Refine some details

Refine mistakes from adaptaton of NetBSD's hardclock man page to
FreeBSD:
o clarify what usermode means
o clarify how often hardclock is called
o remove Xr callout(9) since that's done elsewhere

Reviewed by: mav@
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D30982

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