std::os has documentation for all platforms

Minimum Rust version: 1.21

The std::os module contains operating system specific functionality. You’ll now see more than just linux, the platform we build the documentation on.

We’ve long regretted that the hosted version of the documentation has been Linux-specific; this is a first step towards rectifying that. This is specific to the standard library and not for general use; we hope to improve this further in the future.