Editorial: say "top-level browsing context" in "allowed to use"

This is clearer and is a style used elsewhere, e.g.:
https://html.spec.whatwg.org/#fully-active
https://html.spec.whatwg.org/#allowed-to-navigate

Follow-up to whatwg#1492.

source

@@ -28690,8 +28690,8 @@ interface <dfn>HTMLIFrameElement</dfn> : <span>HTMLElement</span> {
     <li><p>If <var>document</var> has no <span data-x="concept-document-bc">browsing context</span>,
     then return false.</p></li>
 
-    <li><p>If <var>document</var>'s <span data-x="concept-document-bc">browsing context</span> has
-    no <span>browsing context container</span>, then return true.</p></li>
+    <li><p>If <var>document</var>'s <span data-x="concept-document-bc">browsing context</span> is a
+    <span>top-level browsing context</span>, then return true.</p></li>
 
     <li><p>If <var>document</var>'s <span data-x="concept-document-bc">browsing context</span> has a
     <span>browsing context container</span> that is an <code>iframe</code> element with an