Timeline: updated group.content description to show that it can be an element

docs/timeline/index.html

@@ -399,9 +399,9 @@ var groups = [
     </tr>
     <tr>
       <td>content</td>
-      <td>String</td>
+      <td>String or Element</td>
       <td>yes</td>
-      <td>The contents of the group. This can be plain text or html code.</td>
+      <td>The contents of the group. This can be plain text, html code or an html element.</td>
     </tr>
     <tr>
       <td>id</td>