avutil/pixdesc: rewrite AV_PIX_FMT_FLAG_PSEUDOPAL documentation

It seems many people do not understand its current documentation
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavutil/pixdesc.h

@@ -129,10 +129,14 @@ typedef struct AVPixFmtDescriptor {
  * The pixel format contains RGB-like data (as opposed to YUV/grayscale).
  */
 #define AV_PIX_FMT_FLAG_RGB          (1 << 5)
+
 /**
- * The pixel format is "pseudo-paletted". This means that FFmpeg treats it as
- * paletted internally, but the palette is generated by the decoder and is not
- * stored in the file.
+ * The pixel format is "pseudo-paletted". This means that it contains a
+ * fixed palette in the 2nd plane but the palette is fixed/constant for each
+ * PIX_FMT. This allows interpreting the data as if it was PAL8, which can
+ * in some cases be simpler. Or the data can be interpreted purely based on
+ * the pixel format without using the palette.
+ * An example of a pseudo-paletted format is AV_PIX_FMT_GRAY8
  */
 #define AV_PIX_FMT_FLAG_PSEUDOPAL    (1 << 6)
 /**