Use doc_cfg to show feature requirements (#1134)

* Use `doc_cfg` to show feature requirements

* Apply suggestions from code review

src/wasm/response.rs

@@ -77,6 +77,7 @@ impl Response {
 
     /// Try to deserialize the response body as JSON.
     #[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?;