Clarify document.open() event listener removal

Dependency in DOM: whatwg/dom#641.

Tests: web-platform-tests/wpt#10686.

Fixes #2302.

source

@@ -3186,6 +3186,7 @@ a.setAttribute('href', 'https://example.com/'); // change the content attribute
      <li><dfn data-x="dom-Event-initEvent" data-x-href="https://dom.spec.whatwg.org/#dom-event-initevent"><code>initEvent()</code></dfn> method</li>
      <li><dfn data-x-href="https://dom.spec.whatwg.org/#add-an-event-listener">add an event listener</dfn></li>
      <li><dfn data-x="dom-EventTarget-addEventListener" data-x-href="https://dom.spec.whatwg.org/#dom-eventtarget-addeventlistener"><code>addEventListener()</code></dfn> method</li>
+     <li><dfn data-x-href="https://dom.spec.whatwg.org/#remove-all-event-listeners">remove all event listeners</dfn></li>
      <li><dfn data-x="dom-EventListener" data-x-href="https://dom.spec.whatwg.org/#callbackdef-eventlistener"><code>EventListener</code></dfn> callback interface</li>
      <li>The <dfn data-x="concept-event-type" data-x-href="https://dom.spec.whatwg.org/#dom-event-type">type</dfn> of an event</li>
      <li>An <dfn data-x-href="https://dom.spec.whatwg.org/#concept-event-listener">event listener</dfn> and its
@@ -90641,7 +90642,8 @@ document.body.appendChild(frame)</pre>
 
    <li><p><span data-x="abort a document">Abort</span> <var>document</var>.</p></li>
 
-   <li><p>Unregister all event listeners registered on <var>document</var> and its descendants.</p>
+   <li><p>For each <span>shadow-including inclusive descendant</span> <var>node</var> of
+   <var>document</var>, <span>remove all event listeners</span> with <var>node</var>.</p></li>
 
    <li><p>Remove any <span data-x="concept-task">tasks</span> associated with <var>document</var> in
    any <span>task source</span>.</p></li>