Document every publicly visible item and add `#[warn(missing_docs)]`

[bluss]

• add #[warn(missing_docs)]
• Document every publically visible item - until the warnings are fixed
  • Some items might be pub but can be downgraded to pub (crate) - documenting them is then optional

[DrewAlderfer]

Hi, I'm pretty new to Rust, but would like to contribute. This seems like something that I might be able to help on. Is there somewhere that would be a good place to start?