Add more documentation

[wezm]

Currently a lot of the modules are not documented.

[brendanzab]

Handy cargo tip for those not in the know: you can see what the documentation looks like by running cargo doc --open. This makes it relatively easy to see what people will see when they ultimately visit us on docs.rs.

[wezm]

There's still scope for more documentation to be added but a lot has been written since this issue was created, so will close now.