docs: fix build with unstable features

[aschey]

The docs failed to build for 0.22.0 because #[doc(cfg)] is experimental (see build logs here).

[INFO] [stderr] error[E0658]: `#[doc(cfg)]` is experimental
[INFO] [stderr]    --> src/lib.rs:187:20
[INFO] [stderr]     |
[INFO] [stderr] 187 | #[cfg_attr(docsrs, doc(cfg(feature = "wav_output")))]

We can fix this by passing cargo-args = ["-Zunstable-options"]

Additionally, we can use #![cfg_attr(docsrs, feature(doc_cfg))] to automatically document features without having to add #[cfg_attr(docsrs, doc(cfg(feature = "feature_name")))] everywhere.

We can verify this works by running RUSTDOCFLAGS="--cfg docsrs" cargo +nightly doc -Zunstable-options --all-features --no-deps --open and checking the output:

[yara-blue]

docs failed to build for 0.22.0

O.o. 💥
We clearly need that tested in CI. I'll merge this ASAP and put up a new release. Just in case you're a github CI magician could you add a step to CI checking if the docs build?

[yara-blue]

oh and thanks so much for not only reporting but coming up with a fix! 🎉

Especially with something as time sensitive as this 😅

[aschey]

No problem! Here's a follow-up PR to verify docs in CI: #850