add comments to CompletableFutures utility.

Author: cconstable

contrib/temporal-payload-storage-s3/src/main/java/io/temporal/payload/storage/s3/CompletableFutures.java

@@ -7,6 +7,16 @@
 final class CompletableFutures {
   private CompletableFutures() {}
 
+  /**
+   * Returns a future that completes when all of the given futures complete, yielding a list of
+   * their results. If any future completes exceptionally, the returned future also completes
+   * exceptionally with the same exception. If the input list is empty, the returned future completes
+   * immediately with an empty list.
+   *
+   * @param <T>
+   * @param futures
+   * @return
+   */
   static <T> CompletableFuture<List<T>> allOf(List<CompletableFuture<T>> futures) {
     return CompletableFuture.allOf(futures.toArray(new CompletableFuture<?>[0]))
         .thenApply(