hwpmc(4): instead of manual edition, use sysrc(8)

Exemplifying safe use of sysrc(8) has become commonplace in
documentation such as the FreeBSD Handbook. Encourage its use.

Consistency: 'the driver' (not 'this driver').

Co-authored-by: Mina Galić <me+github@igalic.co>
Reviewed-by: imp, Mina Galić <me+github@igalic.co>
Approved by: imp
MFC after: 3 days
Pull-request: https://github.com/freebsd/freebsd-src/pull/785

(cherry picked from commit 666c54d12ec941e1fa3741f28ba19223ec4d2c81)