More explanation for `groupOrder`, see #1241

docs/timeline/index.html

@@ -620,7 +620,8 @@ function (option, path) {
       <td>String or Function</td>
       <td>'order'</td>
       <td>Order the groups by a field name or custom sort function.
-        By default, groups are ordered by an attribute <code>order</code> (if set).
+        By default, groups are ordered by a property <code>order</code> (if set).
+        If no <code>order</code> properties are provided, the order will be undetermined.
       </td>
     </tr>