Editorial: add missing assertion to dialog.show()

Fixes whatwg#10921.

Author: lukewarlow

source

@@ -61924,6 +61924,10 @@ interface <dfn interface>HTMLDialogElement</dfn> : <span>HTMLElement</span> {
    <li><p>Add an <code data-x="attr-dialog-open">open</code> attribute to <span>this</span>, whose
    value is the empty string.</p></li>
 
+   <li><p><span>Assert</span>: <span>this</span>'s <span>node document</span>'s <span>open
+   dialogs list</span> does not <span data-x="list contains">contain</span>
+   <span>this</span>.</p></li>
+
    <li><p>Add <span>this</span> to <span>this</span>'s <span>node document</span>'s <span>open
    dialogs list</span>.</p></li>