Document Subscriber#onNext invocation order

Author: AlexandreCarlton

src/main/java/org/dataloader/BatchPublisher.java

@@ -11,7 +11,8 @@
  * the future returned by {@link DataLoader#load(Object)} to complete as soon as the individual value is available
  * (rather than when all values have been retrieved).
  * <p>
- * It is required that values be returned in the same order as the keys provided.
+ * <b>NOTE:</b> It is <b>required </b> that {@link Subscriber#onNext(V)} is invoked on each value in the same order as
+ * the provided keys.
  *
  * @param <K> type parameter indicating the type of keys to use for data load requests.
  * @param <V> type parameter indicating the type of values returned