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