Further update of the i386 boot documentation

A number of items in the i386 boot documentation have been either
vague, outdated or hard to read.  This text revision adds several more
examples, including a memory map for a modern kernel load.  It also
adds a field-by-field detailed description of the setup header, and a
bootloader ID for Qemu.
Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>