POWER: Clarify ancestor settings objects.

In https://lists.w3.org/Archives/Public/public-webappsec/2015Jul/0011.html,
Boris Zbarsky noted that talking about the incumbent settings object of
an ancestor Document makes no sense. Switching the language to point at
the relevant settings object of each ancestor document's global object.

specs/powerfulfeatures/index.src.html

@@ -49,6 +49,7 @@ <h1>Secure Contexts</h1>
       text: incumbent settings object
       text: responsible document
       text: settings object
+      text: relevant settings object; url: relevant-settings-object-for-a-global-object
   type: attribute
     urlPrefix: webstorage.html
       text: sessionStorage; for: WindowSessionStorage; url: the-sessionstorage-attribute
@@ -421,8 +422,9 @@ <h3 id="settings-secure">
                 <var>ancestor</var>.
               </li>
               <li>
-                If <var>ancestor</var>'s <a>incumbent settings object</a>'s
-                <a>TLS State</a> is <code>Authenticated</code>, skip to the next
+                If the <a>relevant settings object</a> for <var>ancestor</var>'s
+                <a>global object</a> has a <a>TLS State</a> of
+                <code>Authenticated</code>, skip to the next
                 <var>ancestor</var>.
               </li>
               <li>