docs: Enable doc_auto_cfg on docs.rs (#125)

Author: jplatte

src/lib.rs

@@ -34,6 +34,7 @@
 #![doc(
     html_logo_url = "https://raw.githubusercontent.com/smol-rs/smol/master/assets/images/logo_fullsize_transparent.png"
 )]
+#![cfg_attr(docsrs, feature(doc_auto_cfg))]
 
 #[cfg(feature = "std")]
 #[doc(no_inline)]