add dropdown document

lib/ui/tooltip.html

@@ -23,9 +23,11 @@
     <hr />
     <h2>示例</h2>
     <pre>&lt;div id="tooltip-sample"&gt;
+  &lt;!-- 1 --&gt;
   &lt;blockquote title="From MDN Website"&gt;To send an HTTP request, create an XMLHttpRequest object, open a URL, and
     send the request. After the transaction completes, the object will contain useful information such as the
     response body and the HTTP status of the result.&lt;/blockquote&gt;
+  &lt;!-- 2 --&gt;
   &lt;button title="Test to send request through XMLHttpRequest."&gt;Test&lt;/button&gt;
 &lt;/div&gt;