`rustfix`: Add library documentation

[ehuss]

The rustfix library at https://github.com/rust-lang/cargo/tree/master/crates/rustfix/src has very little API or internal documentation. It would be nice if someone could go through and document things and tidy up any existing documentation.

[weihanglo]

ehuss if you're not working on it, I can take a stab.

@rustbot claims

[weihanglo]

@rustbot claim