Fixed #254: unclear description of Timeline.setSelection.

docs/timeline.html

@@ -801,7 +801,7 @@ timeline.clear({options: true}); // clear options only
   <tr>
     <td>setSelection([ids])</td>
     <td>none</td>
-    <td>Select or deselect items. Currently selected items will be unselected.
+    <td>Select one or multiple items by their id. The currently selected items will be unselected. To unselect all selected items, call `setSelection([])`.
     </td>
   </tr>