Write more documentation

[ebkalderon]

At the time of writing, many methods lack documentation and are instead annotated with #[allow(missing_docs)]. These annotations should be replaced with actual doc comments containing explanations and inline doc tests.

Some modules, most notably the top-level lib.rs, might also benefit from an additional section in the existing docs with example code demonstrating proper usage of the API.

[ebkalderon]

Fixed in #81.