Clarification of MACRO equation COST_ENERGY

There have been questions raised for the Taylor formulation of Equation [COST_ENERGY ](https://github.com/iiasa/message_ix/blob/c497a5fd1993d2d63e8c2587b83764471de17d76/message_ix/model/MACRO/macro_core.gms#L273-L290)in MACRO.

- Internal discussion: https://iiasa-ece.slack.com/archives/CD0GBHHA4/p1637231296047600
- External user: https://github.com/iiasa/message_ix/discussions/615

The question is about how the second Taylor coefficient term is derived. We need to clarify how internally and elaborate the conclusion in the documentation.

https://github.com/iiasa/message_ix/blob/c497a5fd1993d2d63e8c2587b83764471de17d76/message_ix/model/MACRO/macro_core.gms#L273-L290
or as rendered in the docs:

Start with the Taylor formulation of COST_ENERGY in message_ix/model/MACRO/macro_core.gms, lines 273–290, then review the linked GitHub Discussion 615 and the internal discussion for the unresolved second-coefficient question. Confirm the derivation and document the conclusion in the rendered MACRO documentation.

Written by the indexing model from the issue text.