lib: enable doc_auto_cfg for docs.rs

The upstream `docs.rs` website uses the nightly release of the Rust
compiler when building project docs. That means we can opt in to the
`doc_auto_cfg` feature to have documentation items automatically mention
the required crate features where appropriate.

This has the advantages of:

* Using a nice styled UI element for the requirements.
* Keeping the feature requirements in-sync with the docs automatically.

Since this feature requires the nightly toolchain we customize the CI
environment so that it is enabled only for the nightly channel.

[0]: https://docs.rs/about/builds

.github/workflows/ci.yml

@@ -58,7 +58,7 @@ jobs:
       - name: cargo doc (all features)
         run: cargo doc --all --all-features --document-private-items
         env:
-          RUSTDOCFLAGS: '-Dwarnings'
+          RUSTDOCFLAGS: ${{ matrix.rust_channel == 'nightly' && '-Dwarnings --cfg=docsrs' || '-Dwarnings' }}
 
   build-windows:
     runs-on: windows-latest

src/lib.rs

@@ -29,6 +29,7 @@ println!("{}", cert.serialize_private_key_pem());
 #![forbid(non_ascii_idents)]
 #![deny(missing_docs)]
 #![allow(clippy::complexity, clippy::style, clippy::pedantic)]
+#![cfg_attr(docsrs, feature(doc_cfg, doc_auto_cfg))]
 
 #[cfg(feature = "pem")]
 use pem::Pem;