pf.conf.5: document af-to (aka nat64)

the patch was started by todd about a year ago and have been
finally finished by phessler and myself today; discussed with
and tweaks from jmc, ok sthen, henning

Obtained from: OpenBSD, mikeb <mikeb@openbsd.org>, 4d5e14dff3
Sponsored by: Rubicon Communications, LLC ("Netgate")