Update aggregation docs to indicate AC vs DC power densities.

[WilliamsTravis]

Why this feature is necessary:
To make the capacity type explicit to help the user enter the correct value for power_density. Currently, wind capacity densities are assumed to be in AC and PV power densities are in DC.

A possible solution is:

1. Add a field that indicates which current the power_density parameter represents.
2. Explain that wind is AC and solar is DC in the documentation.

Charge code
[GDOP.12495.31.01.01] reV Maintenance