hwmon: (fam15h_power) Add documentation for TDP and accumulated power algorithm

This patch adds the description to explain the TDP reporting mechanism
and accumulated power algorithm.

Signed-off-by: Huang Rui <ray.huang@amd.com>
Cc: Borislav Petkov <bp@alien8.de>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>