- new feature `nightly_docs` enables `doc_cfg` unstable feature that allows showing explicit dependencies on features.
- new cargo alias `nightly_docs` for a more convenient local documentation build `cargo +nightly nightly_docs --open`.
- annotate exported items and implementations depending on some feature with the corresponding `doc(cfg())` attribute.
- add the `nightly_docs` feature to the `[package.metadata.docs.rs]` section.
