hwmon: (pmbus/ltc2978) Clean up documentation

Some sysfs attributes are only supported on LTC2978 and not on LTC3880.
Update documentation to reflect which attributes are supported for which chips.

Output current attributes supported on LTC3880 were described as providing
input current values. Fix text to reflect that the attributes provide output
current values.

"reset history" attribute descriptions were misleading and seemed to imply
that all history was reset when writing a single attribute. Replace with more
accurate text.

Replace 'internal temperature" with "chip temperature". Temperature limits
not only apply to the chip temperature, but also to external temperatures
on LTC3880, so remove the word "chip" from the attribute description.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>