Clarify creator origin reuse

Closes #4126.

source

@@ -78416,8 +78416,14 @@ dictionary <dfn>DragEventInit</dfn> : <span>MouseEventInit</span> {
    data-x="concept-url-scheme">scheme</span> is "<code data-x="">javascript</code>", then return
    <var>activeDocumentNavigationOrigin</var>.</p></li>
 
-   <li><p>If <var>invocationOrigin</var> is not null, and <var>url</var> is
-   <code>about:blank</code>, then return <var>invocationOrigin</var>.</p></li>
+   <li>
+    <p>If <var>invocationOrigin</var> is non-null and <var>url</var> is <code>about:blank</code>,
+    then return <var>invocationOrigin</var>.</p>
+
+    <p class="note">The result here is that two documents end up with the same underlying
+    <span>origin</span>, meaning that <code data-x="dom-document-domain">document.domain</code>
+    affects both.</p>
+   </li>
 
    <li><p>If <var>url</var> is <code>about:srcdoc</code>, then return the <span>origin</span> of
    <var>browsingContext</var>'s <span>browsing context container</span>'s <span>node