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