ACPI: watchdog: fix kernel-doc warnings

Fix kernel-doc warnings found when using "W=1".

acpi_watchdog.c:85: warning: This comment starts with '/**', but isn't a kernel-doc comment. Refer Documentation/doc-guide/kernel-doc.rst
acpi_watchdog.c:85: warning: missing initial short description on line:
 * Returns true if this system should prefer ACPI based watchdog instead of

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>