Improve example for TryStreamExt::try_flatten. (#2540)

The previous example was confusing because it didn't show what happened to `Err(4)`. I also added some more examples to show that entries after `Err(_)` are preserved.

Author: kevincox

futures-util/src/stream/try_stream/mod.rs

@@ -736,17 +736,21 @@ pub trait TryStreamExt: TryStream {
     /// thread::spawn(move || {
     ///     tx2.unbounded_send(Ok(2)).unwrap();
     ///     tx2.unbounded_send(Err(3)).unwrap();
+    ///     tx2.unbounded_send(Ok(4)).unwrap();
     /// });
     /// thread::spawn(move || {
     ///     tx3.unbounded_send(Ok(rx1)).unwrap();
     ///     tx3.unbounded_send(Ok(rx2)).unwrap();
-    ///     tx3.unbounded_send(Err(4)).unwrap();
+    ///     tx3.unbounded_send(Err(5)).unwrap();
     /// });
     ///
     /// let mut stream = rx3.try_flatten();
     /// assert_eq!(stream.next().await, Some(Ok(1)));
     /// assert_eq!(stream.next().await, Some(Ok(2)));
     /// assert_eq!(stream.next().await, Some(Err(3)));
+    /// assert_eq!(stream.next().await, Some(Ok(4)));
+    /// assert_eq!(stream.next().await, Some(Err(5)));
+    /// assert_eq!(stream.next().await, None);
     /// # });
     /// ```
     fn try_flatten(self) -> TryFlatten<Self>