Did not document (other than the assembly script that I can't share), although I did post my jail.conf (essentially) in an earlier FreeBSD thread https://news.ycombinator.com/item?id=10602990

If you know how to build a minimal chroot, you know how to build a minimal jail, and then you just need to configure it, etc... If you don't know how to build a minimal chroot, check out option 3 'most minimal chroot possible' here: http://ram.kossboss.com/debian-ubuntu-build-minimal-chroot/

It's written against Debian Linux, but the same thing basically works for FreeBSD, except ldd on FreeBSD omits the loader (/libexec/ld-elf.so.1), but I sort of remembered that from somewhere, and I used truss to see what system calls were being used before it died.