rtp: Correct ff_rtp_get_payload_type documentation

Since 0c378ea1, it can't fail anymore.
Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/rtp.h

@@ -27,9 +27,7 @@
  * Return the payload type for a given codec.
  *
  * @param codec The context of the codec
- * @return In case of unknown payload type or dynamic payload type, a
- * negative value is returned; otherwise, the payload type (the 'PT' field
- * in the RTP header) is returned.
+ * @return The payload type (the 'PT' field in the RTP header).
  */
 int ff_rtp_get_payload_type(AVCodecContext *codec);