stand: Document EFI consoles

Document how EFI consoles work, at least on x86. There's a number of
weird quirks and limitations that are generally known, but not
documented until now. Include information on how EFI decides what the
defualt console is, how to set it and how to cope with common
situations. Note limitations and mismatch between ACPI (which uses UID
to identify a device) and our console code (which uses a raw address)
and explain why we can't translate between them in the loader.

Sponsored by: Netflix
Reviewed by: manu, kevans, rpokala, pauamma
Differential Revision: https://reviews.freebsd.org/D36286