[doc] more detailed explanation for deduplication
Summary:
This is a partial backport of core#23804
https://github.com/bitcoin/bitcoin/pull/23804/commits/9ad211c5753dbd148ba6f0ed56854f6364362ca8
Test Plan: read comment
Reviewers: #bitcoin_abc, Fabien
Reviewed By: #bitcoin_abc, Fabien
Differential Revision: https://reviews.bitcoinabc.org/D12911