x86: documentation fix regarding boot protocol

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: Philipp Kohlbecher <xt28@gmx.de>
Acked-by: Ian Campbell <ijc@hellion.org.uk>
Acked-by: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

Documentation/x86/boot.txt

@@ -537,8 +537,8 @@ Type:		read
 Offset/size:	0x248/4
 Protocol:	2.08+
 
-  If non-zero then this field contains the offset from the end of the
-  real-mode code to the payload.
+  If non-zero then this field contains the offset from the beginning
+  of the protected-mode code to the payload.
 
   The payload may be compressed. The format of both the compressed and
   uncompressed data should be determined using the standard magic