Fix rc_max_rate documentation.

The mpegvideo decoder sets rc_max_rate since 25b7aa98.

libavcodec/avcodec.h

@@ -2290,7 +2290,7 @@ typedef struct AVCodecContext {
     /**
      * maximum bitrate
      * - encoding: Set by user.
-     * - decoding: unused
+     * - decoding: Set by libavcodec.
      */
     int rc_max_rate;