explain chroma_format field in xvmc struct

Originally committed as revision 17239 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/xvmc.h

@@ -42,7 +42,7 @@ struct xvmc_render_state {
     int total_number_of_data_blocks;
     int mc_type; //XVMC_MPEG1/2/4,XVMC_H263 without XVMC_IDCT
     int idct;    //< indicate that IDCT acceleration level is used
-    int chroma_format; //420, 422, 444
+    int chroma_format;  ///< XVMC_CHROMA_FORMAT_420/422/444
     int unsigned_intra; //+-128 for intra pictures after clipping
     XvMCSurface* p_surface; //pointer to rendered surface, never changed