Merge #13662: Explain when reindex-chainstate can be used instead of reindex

Summary:
65a449f8e3 Explain when reindex-chainstate can be used instead of reindex (Sjors Provoost)

Pull request description:

Save users from having to Google this: https://bitcoin.stackexchange.com/a/60711

Tree-SHA512: 3128565d037c77265a2ecf3bce137b8d27740f513802a4e683be06f21a75b82ee6cc22eb903181c4f438a2990cb682ce1d076f4d3af33d5aaa79b783a9f664b1

Backport of Core PR13662
https://github.com/bitcoin/bitcoin/pull/13662/

Test Plan:

make check
./bitcoind --help

Verify help test for -reindex-chainstate.

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

Reviewed By: Fabien, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D4741