Use doc_cfg to show feature requirements (#1134)

* Use `doc_cfg` to show feature requirements

* Apply suggestions from code review

src/async_impl/request.rs

@@ -264,6 +264,7 @@ impl RequestBuilder {
     /// # }
     /// ```
     #[cfg(feature = "multipart")]
+    #[cfg_attr(docsrs, doc(cfg(feature = "multipart")))]
     pub fn multipart(self, mut multipart: multipart::Form) -> RequestBuilder {
         let mut builder = self.header(
             CONTENT_TYPE,
@@ -353,6 +354,7 @@ impl RequestBuilder {
     /// Serialization can fail if `T`'s implementation of `Serialize` decides to
     /// fail, or if `T` contains a map with non-string keys.
     #[cfg(feature = "json")]
+    #[cfg_attr(docsrs, doc(cfg(feature = "json")))]
     pub fn json<T: Serialize + ?Sized>(mut self, json: &T) -> RequestBuilder {
         let mut error = None;
         if let Ok(ref mut req) = self.request {