docs: restore doc_cfg feature gate (#1228)

Author: yuzheng14

src/lib.rs

@@ -2,6 +2,7 @@
 #![deny(unsafe_code)]
 #![deny(missing_docs)]
 #![cfg_attr(test, deny(warnings))]
+#![cfg_attr(docsrs, feature(doc_cfg))]
 
 //! # wreq
 //!