> no, you can also call ntdll (or even syscalls although those aren't stable), there's plenty of documentation on the internet

I mean officially. Yes, you can find some on the internet, but nothing on the official MSDN.

> but also why would you, not much point except for very niche functionality

Exactly. Using system DLL API is backward and forward compatible, and just as standardized and well-documented as the POSIX API. I see no problem relying on it.

(Sidenote: as a bonus, MSDN is surprisingly good, understandable, well organized, lots of examples. I rarely say this, but well done MS, that's how a dev doc should be.)