Guide: added and async fn call example with f64 (#4097)

guide/src/reference/js-promises-and-rust-futures.md

@@ -32,9 +32,13 @@ must be `JsValue` or no return at all:
 ```rust
 #[wasm_bindgen]
 extern "C" {
-    async fn async_func_1() -> JsValue;
+    async fn async_func_1_ret_number() -> JsValue;
     async fn async_func_2();
 }
+
+async fn get_from_js() -> f64 {
+    async_func_1_ret_number().await.as_f64().unwrap_or(0.0)
+}
 ```
 
 The `async` can be combined with the `catch` attribute to manage errors from the