doc: Improve comments describing setBlockIndexCandidates checks

Summary:
The checks are changing slightly in the next commit, so try to explains the
ones that exist to avoid confusioni

This is a partial backport of core#29370
https://github.com/bitcoin/bitcoin/pull/29370/commits/9b97d5bbf980d657a277c85d113c2ae3e870e0ec

Note that the logic is slightly different in our codebase vs Bitcoin Core because of the special case of pindex->nStatus.isOnParkedChain(). For non-parked block everythong should be strictly equivalent

Test Plan: proof-read

Reviewers: #bitcoin_abc, roqqit

Reviewed By: #bitcoin_abc, roqqit

Differential Revision: https://reviews.bitcoinabc.org/D17866