dont block UI if .updateWith() not called (closes #589)

index.html

@@ -2443,11 +2443,6 @@ <h2>
             required by the change. Developers MUST settle the
             <var>detailsPromise</var> to indicate that the payment request is
             valid again.
-              <p>
-                The <a>user agent</a> SHOULD disable any part of the user
-                interface that could cause another update event to be fired.
-                Only one update may be processed at a time.
-              </p>
             </li>
             <li>
               <p>
@@ -2880,9 +2875,17 @@ <h2>
           further action. The <a>user agent</a> user interface should ensure
           that this never occurs.
           </li>
-          <li>
-            <a>Fire an event</a> named <var>name</var> at <var>request</var>
-            using <a>PaymentRequestUpdateEvent</a>.
+          <li>Let <var>event</var> be the result of <a>fire an event</a> named
+          <var>name</var> at <var>request</var> using
+          <a>PaymentRequestUpdateEvent</a>.
+          </li>
+          <li>If <var>event</var>.<a>[[\waitForUpdate]]</a> is true, disable
+          any part of the user interface that could cause another update event
+          to be fired. Wait for <var>event</var>.<a>updateWith()</a> method's
+          <var>detailsPromise</var> to settle.
+          </li>
+          <li>Otherwise, <var>event</var>.<a>[[\waitForUpdate]]</a> is false,
+          then update the user interface based on any changed values.
           </li>
         </ol>
       </section>