Mention that FreeBSD does not ship with /etc/pf.conf by default.

Previous versions of FreeBSD provided a standard /etc/pf.conf, but
that was removed without changing the documentation. Update the
handbook to mention it and point people to the directory
/usr/share/examples/pf/ where example firewall rules are available.

PR: 231977
Submitted by: koobs@
Discussed with: kp@