document and clean up MaybeUpdateMempoolForReorg

Summary:
Co-authored-by: John Newbery <john@johnnewbery.com>

This is a partial backport of core#23976
https://github.com/bitcoin/bitcoin/pull/23976/commits/c7cd98c7176800a51e6a6b3634a26b508aa33ff2

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D12587