hwpmc(4): document debugging options

The debug options for hwpmc are not documented in detail anywhere, and
setting it up was error-prone the first time I had to figure it out (and
each time I've had to remember it). Add some explanation of the required
options and describe the kern.hwpmc.debugflags sysctl format.

Reviewed by: emaste
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D40545