watchdog: document watchdog_init_timeout() wdd parameter

All parameters of watchdog_init_timeout() are documented with exception
of wdd, thus generating a build warning.

This patch document it and so remove the following build warning:
drivers/watchdog/watchdog_core.c:113: warning: No description found for parameter 'wdd'

Signed-off-by: Corentin Labbe <clabbe.montjoie@gmail.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@iguana.be>