Clarify that window.customElements always returns an object

Fixes #11220.

Author: annevk

source

@@ -73696,10 +73696,23 @@ document.body.append(parent);
   map">map</span> of constructors to <code>CustomElementRegistry</code> objects.</p>
 
   <p>The <code>Window</code> <dfn attribute for="Window"><code
-  data-x="dom-window-customElements">customElements</code></dfn> getter steps are to return
-  <span>this</span>'s <span data-x="concept-document-window">associated
-  <code>Document</code></span>'s <span data-x="document-custom-element-registry">custom element
-  registry</span>.</p>
+  data-x="dom-window-customElements">customElements</code></dfn> getter steps are:</p>
+
+  <ol>
+   <li>
+    <p><span>Assert</span>: <span>this</span>'s <span data-x="concept-document-window">associated
+    <code>Document</code></span>'s <span data-x="document-custom-element-registry">custom element
+    registry</span> is a <code>CustomElementRegistry</code> object.</p>
+
+    <p class="note">A <code>Window</code>'s <span data-x="concept-document-window">associated
+    <code>Document</code></span> is always created with a new
+    <code>CustomElementRegistry</code> object.</p>
+   </li>
+
+   <li><p>Return <span>this</span>'s <span data-x="concept-document-window">associated
+   <code>Document</code></span>'s <span data-x="document-custom-element-registry">custom element
+   registry</span>.</p></li>
+  </ol>
 
   <pre><code class="idl">[Exposed=Window]
 interface <dfn interface>CustomElementRegistry</dfn> {