Fix handle any user prompts to set the type for beforeunload prompts (#1816)

Co-authored-by: jgraham <james@hoppipolla.co.uk>

Author: whimboo

index.html

@@ -10645,6 +10645,8 @@ <h3>User Prompt Handler</h3>
 
  <li><p>If the <a>current user prompt</a> is an alert dialog,
  set <var>type</var> to "<code>alert</code>". Otherwise, if
+ the <a>current user prompt</a> is a beforeunload dialog,
+ set <var>type</var> to "<code>beforeUnload</code>". Otherwise, if
  the <a>current user prompt</a> is a confirm dialog,
  set <var>type</var> to "<code>confirm</code>". Otherwise, if
  the <a>current user prompt</a> is a prompt dialog,