hwmon: (lm75) Document why clones are not detected

Explain why clones of the LM75 are generally not detected by the
driver, and why this isn't going to change. Also update the
documentation to reflect the list of chip names currently supported by
the driver.

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