Add a description of the jpeg-xl export options

I spend the whole day on trying to figure out what all these things mean. I still don't know what a Delta-palette is, it's never described in any of the documents. Predictors also took a while, but that greatly simplified when I realized it was the thing that this runs on: https://jxl-art.surma.technology/ I am also unsure if 'quantization' is the same as 'adaptive quantization'.

/cc @lsegovia @alvinwong