ir: Document a bit better the local_id thing.
ir: Document a bit better the local_id thing.

rust-lang/rust-bindgen,emilio/rust-bindgen,emilio/rust-bindgen,rust-lang/rust-bindgen,rust-lang/rust-bindgen,emilio/rust-bindgen,jdub/rust-bindgen,emilio/rust-bindgen,rust-lang/rust-bindgen,jdub/rust-bindgen,jdub/rust-bindgen,emilio/rust-bindgen,jdub/rust-bindgen