Ensure that all public items in the core API are properly documented

There are still a few items in the Rust core that are not documented / are under-documented. We should address these before 1.0.

Checklist:

- [ ] Add module-level documentation where appropriate
- [ ] Add examples where appropriate
- [ ] Resolve any docs issues flagged by `cargo clippy`
- [ ] Check that all public fields and methods have docs