[backport#15267] doc: explain AcceptToMemoryPoolWorker's coins_to_uncache

Summary:
https://github.com/bitcoin/bitcoin/pull/15267/commits/5d262052728acdaa2d108a35ba9921a23b3d761a

Backport of Core PR15267

Test Plan: none

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6819