[PATCH] put a comment at register_die_notifier that the export is used

{un}register_die_notifier() is used by kdb... document this so that future
"remove dead export" rounds can skip this export.

Signed-off-by: Arjan van de Ven <arjan@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>