Add documentation note for designed usecase

futures-util/src/future/always_ready.rs

@@ -40,6 +40,9 @@ impl<T, F: Fn() -> T> Future for AlwaysReady<T, F> {
 
 /// Creates a future that is always immediately ready with a value.
 ///
+/// This is particularly useful in avoiding a heap allocation when an API needs [`Box<dyn Future<Output = T>>`],
+/// as [`AlwaysReady`] does not have to store a boolean for `is_finished`.
+///
 /// # Examples
 ///
 /// ```