pixfmt: extend description for planar YUV 9/10 bits formats

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

libavutil/pixfmt.h

@@ -146,9 +146,11 @@ enum AVPixelFormat {
     AV_PIX_FMT_BGR48BE,   ///< packed RGB 16:16:16, 48bpp, 16B, 16G, 16R, the 2-byte value for each R/G/B component is stored as big-endian
     AV_PIX_FMT_BGR48LE,   ///< packed RGB 16:16:16, 48bpp, 16B, 16G, 16R, the 2-byte value for each R/G/B component is stored as little-endian
 
-    //the following 10 formats have the disadvantage of needing 1 format for each bit depth, thus
-    //If you want to support multiple bit depths, then using AV_PIX_FMT_YUV420P16* with the bpp stored separately
-    //is better
+    /**
+     * The following 12 formats have the disadvantage of needing 1 format for each bit depth.
+     * Notice that each 9/10 bits sample is stored in 16 bits with extra padding.
+     * If you want to support multiple bit depths, then using AV_PIX_FMT_YUV420P16* with the bpp stored separately is better.
+     */
     AV_PIX_FMT_YUV420P9BE, ///< planar YUV 4:2:0, 13.5bpp, (1 Cr & Cb sample per 2x2 Y samples), big-endian
     AV_PIX_FMT_YUV420P9LE, ///< planar YUV 4:2:0, 13.5bpp, (1 Cr & Cb sample per 2x2 Y samples), little-endian
     AV_PIX_FMT_YUV420P10BE,///< planar YUV 4:2:0, 15bpp, (1 Cr & Cb sample per 2x2 Y samples), big-endian