avcodec/avcodec.h: skip_alpha should not be accessed directly (would cause ABI issues)

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/avcodec.h

@@ -2873,6 +2873,7 @@ typedef struct AVCodecContext {
      * However for formats that do not use pre-multiplied alpha
      * there might be serious artefacts (though e.g. libswscale currently
      * assumes pre-multiplied alpha anyway).
+     * Code outside libavcodec should access this field using AVOptions
      *
      * - decoding: set by user
      * - encoding: unused