Windows Desktop has much more documentation than both Android and iOS.
It does take a bit of practice to find things in MSDN, but if it exists^1, then I'll be very surprised if it's not documented. (Except Azure APIs, I don't know why Azure seemed to miss out)
1: By exists I mean a public API. Even some APIs that aren't intended to be used by applications have documentation. Eg: RtlGenRandom.