VCF: usage of ##META lines is unclear

[lbergelson]

In Sample filed format section of vcf 4.3 there's an example of using ##META lines to describe fields in ##SAMPLE lines. It's unclear if these META lines are meant to be treated specially by implementations or if they're just an arbitrary key that's intended to be human readable.

If they're just human readable text, I'm not sure it's worth highlighting them at all. If they're intended to be machine interpretable, then we need much more definition of how they should be used, and whether they're required for the fields in ##SAMPLE lines.