Extend descriptions and comments about the need to create /etc/pf.conf.

FreeBSD removed the default /etc/pf.conf file in previous releases, but
the documentation kept mentioning it like any other file present in the
system. Change pf.conf(5) to mention in the description of the default
ruleset location that this file needs to be created manually. Also, the
default rc.conf file had it's comment extended a bit to let people know
that this file does not exist by default.

PR: 231977
Submitted by: koobs@
Reviewed by: kp@, 0mp@
Approved by: kp@
MFC after: 10 days
Differential Revision: https://reviews.freebsd.org/D19530