describe and make non-optional

Author: fippo

index.bs

@@ -434,7 +434,7 @@ dictionary RTCEncodedAudioFrameMetadata {
     unsigned long synchronizationSource;
     octet payloadType;
     sequence<unsigned long> contributingSources;
-    short? sequenceNumber; // RTP sequence number on incoming frames.
+    short sequenceNumber;
 };
 </pre>
 ### Members ### {#RTCEncodedAudioFrameMetadata-members}
@@ -468,6 +468,15 @@ dictionary RTCEncodedAudioFrameMetadata {
             The list of contribution sources (csrc list) as defined in [[RFC3550]].
         </p>
     </dd>
+    <dt>
+        <dfn>sequenceNumber</dfn> of type <span class=
+            "idlMemberType">short</span>
+    </dt>
+    <dd>
+        <p>
+            The RTP sequence number as defined in [[RFC3550]]. Only exists for incoming audio frames.
+        </p>
+    </dd>
 </dl>
 
 ## <dfn>RTCEncodedAudioFrame</dfn> interface ## {#RTCEncodedAudioFrame-interface}