The file pointed to by the old link 557f6848bd/clippy_lints/src/lib.rs (L110) did not talk about categories and levels. The new link (hopefully) points here https://doc.rust-lang.org/stable/clippy/ which has a nice table explaining the mappings.
557f6848bd/clippy_lints/src/lib.rs (L110)
d7b5cbf065
fdb84cbfd2
This is the source for the Clippy Book. See the book for more information.