pf: add note about changes ported from OpenBSD after 4.5

The last comprehensive sync with OpenBSD's pf corresponds to
OpenBSD 4.5, but many changes have been ported over since then.

Reviewed by: kp, gbe, Kajetan Staszkiewicz <vegeta@tuxpowered.net>
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D42002

(cherry picked from commit 3e609fd481c07802fe837ed834c3710b8c19493c)