Nobody is documenting anything because all that matters for people in this area is for existing C/C++/Rust code to run without modifications on WASM, which mean

Nobody is documenting anything because all that matters for people in this area is for existing C/C++/Rust code to run without modifications on WASM, which means whatever low level POSIX APIs they use is what WASI will support. A large number of software has been ported to run on WASM/WASI, and you can ask yourself how is that possible if WASI doesn't even have a formal API? Because the API is basically being reverse engineered.