divert: Update divert.4 to mention pf

pf's divert-to action can be used to pass packets to a divert socket,
but divert.4 didn't mention this, only referencing ipfw. Try to make
the text a bit more general.

Reviewed by: igoro, kp, glebius
MFC after: 2 weeks
Sponsored by: OPNsense
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D54848