rc.conf(5): Document the 'workstation' firewall_type

Document the workstation ACL ruleset, which uses stateful rules.

While here, add a note about where some of the undocumented variables
can be found. This is not a perfect solution for bug 127359, but it at
at least gives a place to go look, and can be used as a reference for
when bug 127359 gets fixed properly.

PR: 254358, 127359