Merge #13012: [doc] Add comments for chainparams.h, validation.cpp

Summary:
18326ae [doc] Add comments for chainparams.h, validation.cpp (James O'Beirne)

Pull request description:

Added a few comments during a leisurely read through some of the validation code. If this kind of thing seems useful, I can add similar documentation for most of the `CChainState` interface.

Tree-SHA512: a4d9db60383a8ff02e74ac326ed88902eec1ee441e8cd4e1845bcf257072673c15974225288cebf0a633e76a3410f99e2206616b4694725a2a5b0d19c78327d6

Backport of Core PR13012
https://github.com/bitcoin/bitcoin/pull/13012/

Depends on D4045

Test Plan:

make check

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

Reviewed By: deadalnix, jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D4029