Document `pub` functions in `clippy_utils`

Description

Quite a lot of functions in clippy_utils lack documentation. Ideally, the missing_docs lint should be enabled on this crate, except for selected entities such as the static path definitions in the paths module (which might be moved into a submodule).

Note: we are not looking for LLM generated verbose documentation.

Version

Additional Labels

@rustbot label "+good first issue"