Lint missing public API docs

[niklasdewally]

It might be a good idea to lint for missing public API documentation by adding the following to our lib.rs files:

#![warn(missing_docs)]

This could also help us improve our public API by finding things that are public that should instead be private / pub(crate) / #[doc(hidden)].

Given that we have many documentation holes at the moment, we shouldn't add it straight away, but I think in the long term it would be a good check to have in CI.