Document method for reviewers to verify chainTxData

Summary:
This commit adds the final block hash of the window to getchaintxstats
and documents how reviewers can verify changes to chainTxData.

Backport of PR12317: https://github.com/bitcoin/bitcoin/pull/12317/

Completes T566.

Test Plan:
make check
test_runner.py rpc_blockchain

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

Reviewed By: jasonbcox, Fabien, O1 Bitcoin ABC, #bitcoin_abc

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2711