lavf/avidec: Add blurb regarding the skipping of xxpc entries in the index

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/avidec.c

@@ -1581,6 +1581,8 @@ static int avi_read_idx1(AVFormatContext *s, int size)
         st  = s->streams[index];
         ast = st->priv_data;
 
+        /* Skip 'xxpc' palette change entries in the index until a logic
+         * to process these is properly implemented. */
         if ((tag >> 16 & 0xff) == 'p' && (tag >> 24 & 0xff) == 'c')
             continue;