Update docs about fdisk/gdisk support for discoverable partitions

Update docs about fdisk/gdisk support for discoverable partitions

This updates the docs about using fdisk/gdisk to make partitions with the right
partition type UUID as defined in the discoverable partitions spec.

Improve wording/grammar/formatting, and reflect that gdisk supports this as
well nowadays; see https://www.rodsbooks.com/gdisk/revisions.html