Documentation/watchdog: add support for magic close to watchdog-test

Some drivers have the WDIOF_MAGICCLOSE set, which means that applications
need to write 'V' to the watchdog device before closing, otherwise the
driver won't stop the watchdog timer.

Signed-off-by: Timur Tabi <timur@codeaurora.org>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>