avutil/pixfmt: Clarify the meaning of the alpha byte in RGB0 and similar formats

Found-by: wm4
Reviewed-by: wm4 <nfxjfg@googlemail.com>
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Reviewed-by: Carl Eugen Hoyos <cehoyos@ag.or.at>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavutil/pixfmt.h

@@ -255,10 +255,10 @@ enum AVPixelFormat {
     AV_PIX_FMT_BGRA64BE,  ///< packed RGBA 16:16:16:16, 64bpp, 16B, 16G, 16R, 16A, the 2-byte value for each R/G/B/A component is stored as big-endian
     AV_PIX_FMT_BGRA64LE,  ///< packed RGBA 16:16:16:16, 64bpp, 16B, 16G, 16R, 16A, the 2-byte value for each R/G/B/A component is stored as little-endian
 #endif
-    AV_PIX_FMT_0RGB=0x123+4,      ///< packed RGB 8:8:8, 32bpp, 0RGB0RGB...
-    AV_PIX_FMT_RGB0,      ///< packed RGB 8:8:8, 32bpp, RGB0RGB0...
-    AV_PIX_FMT_0BGR,      ///< packed BGR 8:8:8, 32bpp, 0BGR0BGR...
-    AV_PIX_FMT_BGR0,      ///< packed BGR 8:8:8, 32bpp, BGR0BGR0...
+    AV_PIX_FMT_0RGB=0x123+4,///< packed RGB 8:8:8, 32bpp, XRGBXRGB...   X=unused/undefined
+    AV_PIX_FMT_RGB0,        ///< packed RGB 8:8:8, 32bpp, RGBXRGBX...   X=unused/undefined
+    AV_PIX_FMT_0BGR,        ///< packed BGR 8:8:8, 32bpp, XBGRXBGR...   X=unused/undefined
+    AV_PIX_FMT_BGR0,        ///< packed BGR 8:8:8, 32bpp, BGRXBGRX...   X=unused/undefined
     AV_PIX_FMT_YUVA444P,  ///< planar YUV 4:4:4 32bpp, (1 Cr & Cb sample per 1x1 Y & A samples)
     AV_PIX_FMT_YUVA422P,  ///< planar YUV 4:2:2 24bpp, (1 Cr & Cb sample per 2x1 Y & A samples)