improve mimeType definition (in concert with -stats)

Author: fippo

index.bs

@@ -362,7 +362,8 @@ dictionary RTCEncodedVideoFrameMetadata {
     </dt>
     <dd>
         <p>
-            The codec MIME media type/subtype. e.g., video/vp8 or equivalent.
+            The codec MIME media type/subtype defined in the IANA media types registry
+            [[!IANA-MEDIA-TYPES]], e.g. video/VP8.
         </p>
     </dd>
     <dt>
@@ -473,7 +474,8 @@ dictionary RTCEncodedAudioFrameMetadata {
     </dt>
     <dd>
         <p>
-            The codec MIME media type/subtype. e.g., audio/opus or equivalent.
+            The codec MIME media type/subtype defined in the IANA media types registry
+            [[!IANA-MEDIA-TYPES]], e.g. audio/opus.
         </p>
     </dd>
     <dt>