hwmon: (ds1621) Clean up documentation

* The alarms sysfs file is deprecated, and individual alarm files are
  self-explanatory.
* The driver doesn't implement high-reslution temperature readings so
  don't document that.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Cc: Aurelien Jarno <aurelien@aurel32.net>