hwpmc(4): update DEBUGGING section details

The examples I wrote previously do not work. We parse the flags using
explicit names, not the shortened flag suffix. Fix the list of major
flags, and add a compact list of minor flags. Fix the examples, and
tweak some wording for clarity.

Reviewed by: jkoshy, emaste
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Fixes: 5fc97cc32553 ("hwpmc(4): document debugging options")
Differential Revision: https://reviews.freebsd.org/D40913