Add a <warning> that ipfw can lock everyone out if a kernel with ipfw
support is booted without explicitly setting firewall_type=open in
rc.conf or compiling a kernel with IPFIREWALL_DEFAULT_TO_ACCEPT.

Also add a short description of IPFIREWALL_DEFAULT_TO_ACCEPT since we
reference it in the text now. This part will probably need a bit more
work until it's complete.

PR: docs/46747
Submitted by: Lucky Green <shamrock@cypherpunks.to>