docs: some details about system setup and provisioning

In particular, added a description of how the boot order stuff is
supposed to work for EFI now that I actually understand it properly.

(cherry picked from commit a64aec0b2e56171a54be100aa19c75dc441f0249)

Bug: 1030172
Change-Id: I972cda183b27cf3a4cdc76b6a179aa5fb6418d70