Impact: improve documentation

This patch updates the /proc/sys/kernel/nmi_watchdog documentation.
Updated: included Randy Dunlap's corrections.
Signed-off-by: NAristeu Rozanski <aris@redhat.com>
Acked-by: NRandy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>