Clarify mixed ploidy status in spec

[jeromekelleher]

As discussed in sgkit-dev/bio2zarr#179 the current spec is a bit confusing about mixed ploidy genotype calls. Currently we have

A value of -1 indicates missing, and -2 indicates fill in mixed-ploidy datasets.

@timothymillar suggests

A value of -1 indicates missing, and -2 indicates fill value

Although this is somewhat redundant given that we've already defined -1 and -2 for integer fields.

It would perhaps be good to clarify the point that a GT="." is interpreted as a haploid missing call ([-1, -2], if the overall ploidy is 2), where as a true missing diploid call would be "./." ([-1, -1]). Client software should be robust to this, and may want to provide facilities to force full fixed ploidy calls, depending on the VCF producing software and species.