Add documentation to allow #![deny(missing_docs)].

[markcol]

Add documentation to all functions, structs, and types to satisfy the linter when #[deny(missing_docs)] is used. Module-level documentation is still not included for the internal modules since it's really redundant at the moment, so #[deny(missing_docs)] is not currently enabled by default. The documentation for many functions are quite terse and could use additional information to make them accessible for end-users of the library.

[LeopoldArkham]

Thank you, @markcol, it'll look much better this way. I notice you used declarative and imperative forms in the comments; Should we settle on one and use it throughout? I like declarative myself but don't care that much either way.

[LeopoldArkham]

Never mind, all done!