RTCDataChannel: Specify how buffered amount is reduced

webrtc.html

@@ -8912,7 +8912,11 @@ <h2>Attributes</h2>
               increase with each call to the <code><a data-link-for=
               "RTCDataChannel">send()</a></code> method as long as the <a>
               [[\ReadyState]]</a> slot is <code>open</code>; however, the
-              slot does not reset to zero once the channel closes.</p>
+              slot does not reset to zero once the channel closes. When the
+              <a>underlying data transport</a> sends data from its queue, the
+              user agent MUST queue a task that reduces
+              <a>[[\BufferedAmount]]</a> with the number of bytes that was
+              sent.</p>
             </dd>
             <dt><code>bufferedAmountLowThreshold</code> of type <span class=
             "idlAttrType"><a>unsigned long</a></span></dt>