Cleanup outdated error handling docs for unfold

Fixes #1756

Author: cramertj

futures-util/src/stream/unfold.rs

@@ -19,10 +19,6 @@ use pin_utils::{unsafe_pinned, unsafe_unpinned};
 /// will stop producing items and return `Poll::Ready(None)` in future
 /// calls to `poll()`.
 ///
-/// In case of error generated by the returned `Future`, the error will be
-/// returned by the `Stream`.  The `Stream` will then yield
-/// `Poll::Ready(None)` in future calls to `poll()`.
-///
 /// This function can typically be used when wanting to go from the "world of
 /// futures" to the "world of streams": the provided closure can build a
 /// `Future` using other library functions working on futures, and `unfold()`