docs: show the required feature on docs.rs

[EdJoPaTo]

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

While other projects seem to --cfg docsrs in the Cargo.toml this is likely not required (anymore?) as its documented as the way to detect docs.rs

Can be locally previewed:

RUSTDOCFLAGS="--cfg docsrs" cargo +nightly doc --all-features --no-deps --open