pf.conf.5: make "self" a bit more visible

from Sebastian Benoit <benoit-lists at fb12.de>, ok/input jmc, reminder/input
deraadt and too much of a trail to mention all of it, thx everybody involved

Obtained from: OpenBSD, henning <henning@openbsd.org>, 682c71ec7c
Sponsored by: Rubicon Communications, LLC ("Netgate")