Revert change to ActivateBestChain() comments made in D1182

Summary: Reverts changes to the comments for ActivateBestChain() in validation.cpp that were made in D1182. This reduces our code ownership and reduces merge conflicts.

Test Plan:

make check

Reviewers: deadalnix, Fabien, jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D4045