watchdog: docs: kernel-api: don't reference removed functions

Those functions are not only deprecated, but gone for good.

Fixes: 62cd1c40

 ("watchdog: kill unref/ref ops")
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.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>