arch/*: Kconfig: fix documentation for NMI watchdog

Changeset 9919cba7 ("watchdog: Update documentation") updated
the documentation, removing the old nmi_watchdog.txt and adding
a file with a new content.

Update Kconfig files accordingly.

Fixes: 9919cba7 ("watchdog: Update documentation")

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Acked-by: Jonathan Corbet <corbet@lwn.net>