lib: add JSDoc to os module functions

Inspired by @bmeck: https://twitter.com/bradleymeck/status/1380643627211354115

I only pulled in actual docs if it seemed relevant, but can add more. It seemed redundant to copy the docs wholesale, but I can!

I also wasn't sure what the preferred style for types was with regards to x[] vs Array<x>, so I went with the recommended array-simple mode of typescript-eslint.

Some screenshots: