For version 3, here is the best I have found so far :
http://james.apache.org/server/3/config.html (not very helpful, but it is something)
http://www.mail-archive.com/server-user@james.apache.org/ (see questions of others)
https://james.apache.org/mail.html (mailing lists - when no docs are available, bugging people may help or force them to write more documentation... Although even that doesn't seem to work with Apache...)
If I manage to make it work the way I want, I might put some instructions here as well.