Add documentation to public structs/functions/macros

We can achieve this by adding
```rust
#![deny(missing_docs)]
```
To the top of crates' `lib.rs`/`main.rs` file, building and fixing all of the places where it complains.

Locate each crate's lib.rs or main.rs and add the proposed missing-docs lint before building. Follow the resulting diagnostics across the crates and add documentation wherever reported; done means the build completes without missing-documentation errors.

Written by the indexing model from the issue text.