hwmon: Clarify scope of attribute access

Hardware monitoring sysfs attributes are used and displayed by unrestricted
userspace applications. Standard attributes therefore have to be world
readable, since otherwise those userspace applications would either have
to run as super-user or display an error. None of those makes sense.
Clarify the expected scope of attribute access in the ABI document.

Cc: Naveen Krishna Chatradhi <nchatrad@amd.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>