Editorial: describe permissions policy reliance in priv/sec section (#…

…210)

* Editorial: add <p> to Priv/Sec about 'allowed to use'

* tidy

* Reword permissions policy text

index.html

@@ -509,6 +509,10 @@ <h2>
         is only [=exposed=] in [=secure contexts=] (such as `https://`
         schemes).
         </li>
+        <li>Developers can use the means afforded by the
+        [[[permissions-policy]]] specification to control if and when a
+        third-party context is [=allowed to use=] this API.
+        </li>
         <li>Use of {{Navigator/share()}} from a <a href=
         "https://en.wikipedia.org/wiki/Privacy_mode">private browsing mode</a>
         might leak private data to a third-party application that does not