Joat has begun to document the the Nintendo DS’s 3D hardware because, as he put it, “no-one else has gotten around to it”:

Since no-one else has gotten around to it, I started documenting the 3D hardware (Geometry Engine, Rasterizing Engine). It is still a work in progress.
Geometry Engine Documentation can be viewd [here].

Rasterizing Engine Documentation can be viewed [here].