Use `doc_cfg` once stabilized

Following up from #2223, once https://github.com/rust-lang/rust/issues/43781 stabilizes we should use whatever it requires instead of `#![cfg_attr(docsrs, feature(doc_cfg))]`.

Start by reading the discussion in #2223 and check Rust issue #43781 to confirm whether the required `doc_cfg` feature has stabilized and what it requires. Then locate the repository's `#![cfg_attr(docsrs, feature(doc_cfg))]` usage and update it according to the stabilized requirement; done means the obsolete feature usage is removed or replaced and documentation builds successfully.

Written by the indexing model from the issue text.