@Async docs explicitly mention ListenableFuture and CompletableFuture

Issue: SPR-14881

spring-context/src/main/java/org/springframework/scheduling/annotation/Async.java

@@ -29,12 +29,18 @@ import java.lang.annotation.Target;
  *
  * <p>In terms of target method signatures, any parameter types are supported.
  * However, the return type is constrained to either {@code void} or
- * {@link java.util.concurrent.Future}. In the latter case, the {@code Future} handle
- * returned from the proxy will be an actual asynchronous {@code Future} that can be used
- * to track the result of the asynchronous method execution. However, since the
- * target method needs to implement the same signature, it will have to return
- * a temporary {@code Future} handle that just passes the return value through: e.g.
- * Spring's {@link AsyncResult} or EJB 3.1's {@link javax.ejb.AsyncResult}.
+ * {@link java.util.concurrent.Future}. In the latter case, you may declare the
+ * more specific {@link org.springframework.util.concurrent.ListenableFuture} or
+ * {@link java.util.concurrent.CompletableFuture} types which allow for richer
+ * interaction with the asynchronous task and for immediate composition with
+ * further processing steps.
+ *
+ * <p>A {@code Future} handle returned from the proxy will be an actual asynchronous
+ * {@code Future} that can be used to track the result of the asynchronous method
+ * execution. However, since the target method needs to implement the same signature,
+ * it will have to return a temporary {@code Future} handle that just passes a value
+ * through: e.g. Spring's {@link AsyncResult}, EJB 3.1's {@link javax.ejb.AsyncResult},
+ * or {@link java.util.concurrent.CompletableFuture#completedFuture(Object)}.
  *
  * @author Juergen Hoeller
  * @author Chris Beams