Added an extra note.

Author: mgiuca

index.html

@@ -1981,6 +1981,11 @@ <h3>
           <li>Otherwise, return <var>start URL</var>.
           </li>
         </ol>
+        <div class="note">
+          The default start URL (if <code>start_url</code> is omitted or an
+          error) is the <var>manifest URL</var>, with its filename, query, and
+          fragment removed.
+        </div>
         <div class="example">
           <p>
             For example, if the value of <a>start_url</a> is