I've started to generate some documentation using Doxygen; it's now available at http://www.themoebius.org.uk/kernel/.
The documentation consists of a reference to the various parts of the Mbius kernel and a driver writer's howto giving the steps required to get a Mbius device driver up and running. Examples of device drivers are included in the Mbius source code.
More documentation, including a more full reference to the kernel, additional howtos, and a user library reference, will become available as I write it. Remember that the Mbius page also features a more general set of Operating System Design tutorials (http://www.themoebius.org.uk/tutes/).
Feel free to send me any comments, questions or suggestions on any of this.