mod_cloud_notify_extensions: Add note about dependency
Noting here because people might not click through to see it on the
mod_cloud_notify_encrypted page.
<tr>
<th>What is {x} {op} {y}?</th><td>
<input name="captcha_challenge" type="hidden" value="{challenge}">
<input name="captcha_reply" pattern="[0-9]+" required type="number">
</td></tr>