pf.conf.5: document received-on

Reviewed by: zlei
Obtained from: OpenBSD, dlg <dlg@openbsd.org>, 456093dddc
Obtained from: OpenBSD, jmc <jmc@openbsd.org>, 2bf0158fa8
Obtained from: OpenBSD, deraadt <deraadt@openbsd.org>, be785dc6e2
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D46579