Impact: clarify documentation

Documentation/x86/boot.txt describes payload_offset as the offset
from the end of the real-mode code. In fact, it is more accurately
described as the offset from the beginning of the protected-mode
code, as (a) this is how it is actually calculated and (b) the padding
after the real-mode code is not included in the offset.
Signed-off-by: NPhilipp Kohlbecher <xt28@gmx.de>
Acked-by: NIan Campbell <ijc@hellion.org.uk>
Acked-by: NH. Peter Anvin <hpa@zytor.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>