Revise public documentation for codegen

[jeffcharles]

Description of the change

Revising some of the public documentation for the codegen crate and some of the other project docs.

Why am I making this change?

These are admittedly somewhat nitpicky changes but I would like to see them adopted before publishing the crate.

Checklist

• I've updated the relevant CHANGELOG files if necessary. Changes to javy-cli and javy-plugin do not require updating CHANGELOG files.
• I've updated the relevant crate versions if necessary. Versioning policy for library crates
• I've updated documentation including crate documentation if necessary.