Documentation: power: include kernel-doc in Energy Model doc

Improve the existing documentation of the Energy Model and add kernel-doc
comments. This extends an API description and helps better understanding
the usage.

Signed-off-by: Lukasz Luba <lukasz.luba@arm.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Author: lukaszluba-arm

Documentation/power/energy-model.rst

@@ -101,8 +101,7 @@ subsystems which use EM might rely on this flag to check if all EM devices use
 the same scale. If there are different scales, these subsystems might decide
 to: return warning/error, stop working or panic.
 See Section 3. for an example of driver implementing this
-callback, and kernel/power/energy_model.c for further documentation on this
-API.
+callback, or Section 2.4 for further documentation on this API
 
 
 2.3 Accessing performance domains
@@ -123,7 +122,17 @@ em_cpu_energy() API. The estimation is performed assuming that the schedutil
 CPUfreq governor is in use in case of CPU device. Currently this calculation is
 not provided for other type of devices.
 
-More details about the above APIs can be found in include/linux/energy_model.h.
+More details about the above APIs can be found in ``<linux/energy_model.h>``
+or in Section 2.4
+
+
+2.4 Description details of this API
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+.. kernel-doc:: include/linux/energy_model.h
+   :internal:
+
+.. kernel-doc:: kernel/power/energy_model.c
+   :export:
 
 
 3. Example driver