grub_device option not documented for partitions

[Jonathan Heathcote]

The grub_device storage option may be used for both disks and partitions however the documentation does not mention that the option is also supported for partitions.

See disk documentation (which includes 'grub_device' documentation)

https://curtin.readthedocs.io/en/latest/topics/storage.html?highlight=grub_device#disk-command

And partition documentation (which omits any mention of it):

https://curtin.readthedocs.io/en/latest/topics/storage.html?highlight=grub_device#partition-command

As an example of this options' importance: for UEFI based systems, the grub_device option is applied to the UEFI system partition rather than the disk as a whole.

See also definition of grub_device as a valid key for partitions in block/schemas.py:314