Remove a Document from its session history entry when it's discarded

As implied by the Note after the definition of session history entries.

Author: jyasskin

source

@@ -78947,6 +78947,10 @@ dictionary <dfn>WindowPostMessageOptions</dfn> : <span>PostMessageOptions</span>
    <li><p><span data-x="a browsing context is discarded">Discard</span> all the <span data-x="child
    browsing context">child browsing contexts</span> of <var>document</var>.</p></li>
 
+   <li><p>For each <span>session history entry</span> <var>entry</var> with a <code>Document</code>
+   object equal to <var>document</var>, remove <var>entry</var>'s <code>Document</code>
+   object.</p></li>
+
    <li><p>Lose the strong reference from <var>document</var>'s <span
    data-x="concept-document-bc">browsing context</span> to <var>document</var>.</p></li>