watchdog: hpwdt: Update Driver Documentation.

Remove references to deprecated features like NMI sourcing
and obsoleted module parameters.

Add details concerning new module parameter pretimeout and tips
to programming it.

Signed-off-by: Jerry Hoemann <jerry.hoemann@hpe.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>