Document all the various API bits.

[Pomax]

There's quite a bit to document, most of it "obviously" named, but some of it not. We need to get some API documentation going.

Ideally this would be autogenerated from source using something like jsdoc, but only if that's not crazily overly verbose.

[Pomax]

https://medium.com/swlh/creating-better-jsdoc-documentation-8b7a65744dcb