hwmon: Document the sensor enable attribute

Document new ABI attributes:
	in[0-*]_enable
	fan[1-*]_enable
	temp[1-*]_enable
	curr[1-*]_enable
	power[1-*]_enable
	energy[1-*]_enable
	humidity[1-*]_enable

Signed-off-by: Shilpasri G Bhat <shilpa.bhat@linux.vnet.ibm.com>
[groeck: Updated commit text, removed unnecessary details from
 ABI description]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>