make note about timestamp-yness

Author: fippo

index.bs

@@ -289,7 +289,7 @@ dictionary RTCEncodedVideoFrameMetadata {
 [Exposed=(Window,DedicatedWorker)]
 interface RTCEncodedVideoFrame {
     readonly attribute RTCEncodedVideoFrameType type;
-    readonly attribute unsigned long timestamp;
+    readonly attribute unsigned long timestamp; // RTP timestamp.
     attribute ArrayBuffer data;
     RTCEncodedVideoFrameMetadata getMetadata();
 };
@@ -301,7 +301,7 @@ dictionary RTCEncodedAudioFrameMetadata {
 
 [Exposed=(Window,DedicatedWorker)]
 interface RTCEncodedAudioFrame {
-    readonly attribute unsigned long timestamp;
+    readonly attribute unsigned long timestamp; // RTP timestamp.
     attribute ArrayBuffer data;
     RTCEncodedAudioFrameMetadata getMetadata();
 };