Anyway, I proposed myself to deal with the documentation whenever I had enough free time to rewrite most of it and restructure its sections to make it somewhat decent. And guess what, this is what I started to do some days (a week?) ago. So... here is the so-promised documentation!
Be aware that this is still just for review. The documentation will end up either being part of The NetBSD Guide or being a "design and implementation" guide on its own.
Also note that there is still much work to do. Many sections are not yet written. In fact, I started writing the general ideas to get into file system development because, once you know the basics, learning other stuff is relatively easy by looking at existing manual pages and code. Of course, the document should eventually be completed, specially to avoid having to reverse-engineer code.
I'll seize this post to state something: the lack of documentation is a serious problem in most open source projects, specially those that have some kind of extensibility. Many developers don't like to write documentation and, what is worse, they think it's useless, that other developers will happily read the existing code to understand how things work. Wrong! If you are willing to extend some program, you want its interface to be clearly defined and documented; if there is no need to look at the code (except for carefully done examples), even better. FWIW, reading the program's code can be dangerous because you may get things wrong and end up relying on implementation details. So, write documentation, even if it is tough and difficult (I know, it can be very difficult, but don't you like challenges? ;-).