Code review comment for ~dbungert/curtin:partition-arbitrary-types

> 1) needs documentation
Done. I debated leaving it undocumented on purpose as too obscure of a detail, but this is fine.

2) it should work with gpt partition tables too?
Done.

3) is it ok that this only works with v2 configs? (probably...)
That's my intention.

Also added disk label ID preservation, that's needed.