refactor: Post-"Chainstate loading sequence coalescence" fixups

Summary:

docs: Make LoadChainstate comment more accurate

style-only: Rename *Chainstate return values

init: Use clang-tidy named args syntax

This is a backport of core#23855

Depends on D14584

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D14585