doc: remove potentially confusing ChainstateManager comment
doc: remove potentially confusing ChainstateManager comment

Summary:
This concludes backport of [[https://github.com/bitcoin/bitcoin/pull/19806 | 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

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