docs: add returning on the first error example for try_join! (#4133)

tokio/src/macros/try_join.rs

@@ -59,6 +59,45 @@
 ///     }
 /// }
 /// ```
+///
+/// Using `try_join!` with spawned tasks.
+///
+/// ```
+/// use tokio::task::JoinHandle;
+///
+/// async fn do_stuff_async() -> Result<(), &'static str> {
+///     // async work
+/// # Err("failed")
+/// }
+///
+/// async fn more_async_work() -> Result<(), &'static str> {
+///     // more here
+/// # Ok(())
+/// }
+///
+/// async fn flatten<T>(handle: JoinHandle<Result<T, &'static str>>) -> Result<T, &'static str> {
+///     match handle.await {
+///         Ok(Ok(result)) => Ok(result),
+///         Ok(Err(err)) => Err(err),
+///         Err(err) => Err("handling failed"),
+///     }
+/// }
+///
+/// #[tokio::main]
+/// async fn main() {
+///     let handle1 = tokio::spawn(do_stuff_async());
+///     let handle2 = tokio::spawn(more_async_work());
+///     match tokio::try_join!(flatten(handle1), flatten(handle2)) {
+///         Ok(val) => {
+///             // do something with the values
+///         }
+///         Err(err) => {
+///             println!("Failed with {}.", err);
+///             # assert_eq!(err, "failed");
+///         }
+///     }
+/// }
+/// ```
 #[macro_export]
 #[cfg_attr(docsrs, doc(cfg(feature = "macros")))]
 macro_rules! try_join {