x86, boot: Update comments about entries for 64bit image

Now 64bit entry is fixed on 0x200, can not be changed anymore.

Update the comments to reflect that.

Also put info about it in boot.txt

-v2: fix some grammar error

Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Link: http://lkml.kernel.org/r/1359058816-7615-27-git-send-email-yinghai@kernel.org
Cc: Rob Landley <rob@landley.net>
Cc: Matt Fleming <matt.fleming@intel.com>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>