message: Add doc comment to explain wasi-generator-extra
diff: +//! This generator is run when regenerate.sh is executed and fixes a couple
  +//! of issues that wit-bindgen currently doesn't support.
  +//!
  +//! Eventually this functionality should be upstreamed into wit-bindgen,
  +//! see issue [#3177](https://github.com/wasmerio/wasmer/issues/3177).
  +
  use convert_case::{Case, Casing};
  use wit_parser::TypeDefKind;
  