update afterCursor javadoc

gcloud-java-datastore/src/main/java/com/google/gcloud/datastore/QueryResults.java

@@ -35,7 +35,21 @@ public interface QueryResults<V> extends Iterator<V> {
   Class<?> resultClass();
 
   /**
-   * Returns the Cursor for point after the value returned in the last {@link #next} call.
+   * Returns the Cursor for the point after the value returned in the last {@link #next} call. This
+   * cursor can be used to issue subsequent queries (with the same constraints) that may return
+   * additional results.
+   *
+   * <p>A simple use case:
+   * <pre> {@code
+   * Query<Entity> query = Query.entityQueryBuilder()
+   *     .kind("Person")
+   *     .filter(PropertyFilter.eq("favoriteFood", "pizza"))
+   *     .build();
+   * QueryResults<Entity> results = datastore.run(query);
+   * // Consume some results (using results.next()) and do any other actions as necessary.
+   * query = query.toBuilder().startCursor(results.cursorAfter()).build();
+   * results = datastore.run(query); // now we will iterate over all entities not yet consumed
+   * </pre>
    */
   Cursor cursorAfter();
 }