- Clarify that YUV encoding performs downsampling as well as color
conversion.
- Clarify that TJPARAM_LOSSLESS is ignored by tj3CompressFromYUV*8().
- Clarify that tj3CompressFromYUV*8() generates only YCbCr or grayscale
JPEG images, i.e. that TJPARAM_COLORSPACE has no effect.