pmc: clarify and strengthen statements about API/ABI

• Make it clear that applications are not only encouraged to use the pmc(3) library, but use of the hwpmc(4) interface directly is a use-case we do not support
• Move the COMPATIBILITY section above PROGRAMMING API in hwpmc(4)
• Drop statements about the driver and library being "under development"

Reviewed by: jkoshy, emaste
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D40543

(cherry picked from commit 3977781b987e369a9fe23bb642b9680b0a3887c1)