lavc: docs: the field_order is set by the demuxer and not libavcodec for decoding

This makes the documentation match the implementation
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/avcodec.h

@@ -2225,7 +2225,7 @@ typedef struct AVCodecContext {
 
     /** Field order
      * - encoding: set by libavcodec
-     * - decoding: Set by libavcodec
+     * - decoding: Set by user.
      */
     enum AVFieldOrder field_order;