pf: update pf(4) man page to list DIOCGETSTATESV2

The nvlist based state retrieval ioctl has been replaced by an old-style
ioctl for performance reasons. Document that one.

Reported by: Michael Gmelin <grembo@freebsd.org>
MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D42331