Use doc_cfg to show feature requirements (#1134)

* Use `doc_cfg` to show feature requirements

* Apply suggestions from code review

src/async_impl/response.rs

@@ -103,6 +103,7 @@ impl Response {
     ///
     /// This requires the optional `cookies` feature to be enabled.
     #[cfg(feature = "cookies")]
+    #[cfg_attr(docsrs, doc(cfg(feature = "cookies")))]
     pub fn cookies<'a>(&'a self) -> impl Iterator<Item = cookie::Cookie<'a>> + 'a {
         cookie::extract_response_cookies(&self.headers).filter_map(Result::ok)
     }
@@ -236,6 +237,7 @@ impl Response {
     ///
     /// [`serde_json::from_reader`]: https://docs.serde.rs/serde_json/fn.from_reader.html
     #[cfg(feature = "json")]
+    #[cfg_attr(docsrs, doc(cfg(feature = "json")))]
     pub async fn json<T: DeserializeOwned>(self) -> crate::Result<T> {
         let full = self.bytes().await?;
 
@@ -308,6 +310,7 @@ impl Response {
     ///
     /// This requires the optional `stream` feature to be enabled.
     #[cfg(feature = "stream")]
+    #[cfg_attr(docsrs, doc(cfg(feature = "stream")))]
     pub fn bytes_stream(self) -> impl futures_core::Stream<Item = crate::Result<Bytes>> {
         self.body
     }