UPDATING: update notes on EFI booting

The notes on EFI booting and updating for ZFS had become dated and only
partially updated. Expand the notes with a few more details and a
pointer to laoder.efi(8) and uefi(8).

Sponsored by: Netflix
Discussed with: pauamma, karels
Differential Revision: https://reviews.freebsd.org/D36629