hwpmc(4): more detailed SYNOPSYS

Include some boilerplate similar to other section 4 man pages,
describing how to load the module at boot-time or include it in the
kernel.

Reviewed by: jkoshy, emaste
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D40542

(cherry picked from commit 0a1a48148ccd561464d643c05597cec2ae9e88b8)