hwmon: (w83627ehf) Describe undocumented pwm attributes

Add description of pwm[1-4]_start_output, pwm[1-4]_step_output,
pwm[1-4]_stop_output, and pwm[1-4]_max_output attributes to driver
documentation.

Signed-off-by: Guenter Roeck <guenter.roeck@ericsson.com>
Acked-by: Jean Delvare <khali@linux-fr.org>