doc: remove potentially confusing ChainstateManager comment
Summary:
This concludes backport of core#19806 [6/6]
https://github.com/bitcoin/bitcoin/pull/19806/commits/1afc0e4aa1b910991d4f8a77d74e2197f370987c
Test Plan: NA
Reviewers: #bitcoin_abc, Fabien
Reviewed By: #bitcoin_abc, Fabien
Differential Revision: https://reviews.bitcoinabc.org/D11243