Update documentation for flatten
@@ -652,7 +652,7 @@ pub trait Future {
/// `IntoFuture` trait and the error can be created from this future's error
/// type.
///
- /// This method is equivalent to `self.then(|x| x)`.
+ /// This method is roughly equivalent to `self.and_then(|x| x)`.
///
/// Note that this function consumes the receiving future and returns a
/// wrapped version of it.
