[backport#15531] Merge #15531: Suggested interfaces::Chain cleanups from #15288

Summary:
4d4e4c6448 Suggested interfaces::Chain cleanups from #15288 (Russell Yanofsky)

Pull request description:

Mostly documentation improvements requested in the last review of #15288 before it was merged (https://github.com/bitcoin/bitcoin/pull/15288#pullrequestreview-210241864)

Tree-SHA512: 64e912520bbec20a44032f265a8cf3f11ad7f5126c8626b5ad5e888227b1f92ecb321522fab4bbbd613230b55450abd6ace023631d0a4f357a780d65c5638bfe

Depends on D5869

This is a backport of Core PR15531

Test Plan:

cmake .. -GNinja -DENABLE_WERROR=ON -DCMAKE_BUILD_TYPE=Debug
ninja check

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5870