Generate docs with features for docs.rs

Generate docs with features for docs.rs

Enable generating docs using --all-features or --features="std" where
applicable. Additionally, use doc_auto_cfg to tag items requiring a
feature.

https://doc.rust-lang.org/nightly/rustdoc/unstable-features.html#doc_auto_cfg-automatically-generate-doccfg

This requires building with nightly, which is what is used by docs.rs.

https://docs.rs/about/builds

To test locally, use:

RUSTDOCFLAGS="--cfg docsrs" cargo +nightly doc ...