Documentation/i386/boot.txt: update and correct

In the process of rewriting the x86 setup code, I found a number of
inaccuracies and outdated recommendations in the boot protocol
documentation.  Revamp to make it more up to date.

In particular, the common use of the heap actually requires (slightly)
more than 4K of heap plus stack, which is the recommended amount in
the document; currently the code compensates by being smaller than
specified, but we can't assume that will be true forever.  Thus,
recommend that if we have a modern bzImage kernel, that the bootloader
maximizes the available space.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>