Document flush behavior on error for send_all & forward (#2404)

Signed-off-by: ktf <krunotf@gmail.com>

Author: ktff

futures-util/src/sink/mod.rs

@@ -243,7 +243,8 @@ pub trait SinkExt<Item>: Sink<Item> {
     /// This future will drive the stream to keep producing items until it is
     /// exhausted, sending each item to the sink. It will complete once both the
     /// stream is exhausted, the sink has received all items, and the sink has
-    /// been flushed. Note that the sink is **not** closed.
+    /// been flushed. Note that the sink is **not** closed. If the stream produces 
+    /// an error, that error will be returned by this future without flushing the sink.
     ///
     /// Doing `sink.send_all(stream)` is roughly equivalent to
     /// `stream.forward(sink)`. The returned future will exhaust all items from

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

@@ -1329,7 +1329,8 @@ pub trait StreamExt: Stream {
     /// the sink is closed. Note that neither the original stream nor provided
     /// sink will be output by this future. Pass the sink by `Pin<&mut S>`
     /// (for example, via `forward(&mut sink)` inside an `async` fn/block) in
-    /// order to preserve access to the `Sink`.
+    /// order to preserve access to the `Sink`. If the stream produces an error,
+    /// that error will be returned by this future without flushing/closing the sink.
     #[cfg(feature = "sink")]
     #[cfg_attr(docsrs, doc(cfg(feature = "sink")))]
     fn forward<S>(self, sink: S) -> Forward<Self, S>