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