add 'selector' option motivation to tooltip docs

docs/_includes/js/tooltips.html

@@ -124,7 +124,7 @@ $('#example').tooltip(options)
          <td>selector</td>
          <td>string</td>
          <td>false</td>
-         <td>If a selector is provided, tooltip objects will be delegated to the specified targets.</td>
+         <td>If a selector is provided, tooltip objects will be delegated to the specified targets. In practice, this is used to enable dynamic HTML content to have tooltips added. See <a href="https://github.com/twbs/bootstrap/issues/4215">this</a> and <a href="http://jsfiddle.net/fScua/">an informative example</a>.</td>
        </tr>
        <tr>
          <td>template</td>