x86 NOTES: Move shared options from amd/i386 NOTES to x86 NOTES

While here, reorder some of the entries using headers more aligned
with sys/conf/NOTES. Also add a pointer from the amd64/i386 NOTES
files to x86 NOTES.

The "extra" ACPI device drivers were only present in i386 NOTES
previously.

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D44787