docs(lib): enable doc_cfg on docs.rs builds (#2372)

.github/workflows/CI.yml

@@ -132,4 +132,4 @@ jobs:
         uses: actions-rs/cargo@v1
         with:
           command: rustdoc
-          args: --features full -- -D broken-intra-doc-links
+          args: --features full -- --cfg docsrs -D broken-intra-doc-links

src/lib.rs

@@ -4,6 +4,7 @@
 #![cfg_attr(test, deny(rust_2018_idioms))]
 #![cfg_attr(test, deny(warnings))]
 #![cfg_attr(all(test, feature = "nightly"), feature(test))]
+#![cfg_attr(docsrs, feature(doc_cfg))]
 
 //! # hyper
 //!