chain: Rename HaveTxsDownloaded to HaveNumChainTxs, add note

Summary:
This is a partial backport of core#27596 and core#28562
https://github.com/bitcoin/bitcoin/pull/27596/commits/99839bbfa7110c7abf22e587ae2f72c9c57d3c85
https://github.com/bitcoin/bitcoin/pull/28562/commits/a482f86779a6182d87004b463c0eaf21038181c3

Test Plan: ninja all check-all bitcoin-fuzzers

Reviewers: #bitcoin_abc, roqqit

Reviewed By: #bitcoin_abc, roqqit

Differential Revision: https://reviews.bitcoinabc.org/D17864