w83791d: Documentation update

The alarm bits and the beep enable bits are in different positions in
the hardware. Document the problem and leave it to the user-space code
to handle the situation. When this driver is updated to the standardized
sysfs alarm/beep methodology, this won't be a problem.

This is a documentation only change.

Signed-off by: Charles Spirakis <bezaur@gmail.com>
Signed-off-by: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>