watchdog: clarify that stop() is optional

The commit d0684c8a ("watchdog: Make stop function optional")
made stop function not mandatory, but the comments
and the doc weren't reflected. Fix it to clarify.

Signed-off-by: Bumsik Kim <k.bumsik@gmail.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20200403031507.63487-1-k.bumsik@gmail.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>