drm/amdgpu: add documentation on hwmon interfaces exposed (v3)

Provide detail on the currently exposed hwmon interfaces
for temperature, power, voltage, and fan.

v2: add power cap documentation
v3: add a comment about sensors tool

Reviewed-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>