compatibility warning

Originally committed as revision 6035 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavutil/avutil.h

@@ -70,7 +70,7 @@ enum PixelFormat {
     PIX_FMT_XVMC_MPEG2_IDCT,
     PIX_FMT_UYVY422,   ///< Packed YUV 4:2:2, 16bpp, Cb Y0 Cr Y1
     PIX_FMT_UYYVYY411, ///< Packed YUV 4:1:1, 12bpp, Cb Y0 Y1 Cr Y2 Y3
-    PIX_FMT_NB,
+    PIX_FMT_NB,        ///< number of pixel formats, DO NOT USE THIS if you want to link with shared libav* because the number of formats might differ between versions
 };
 
 #ifdef WORDS_BIGENDIAN