AcceptPackage fixups

Summary:
No behavior changes, just clarifications.

This is a partial backport of core#23804
https://github.com/bitcoin/bitcoin/pull/23804/commits/9d88853e0c85f765f7d982b15e8122ede50110ed

Depends on D12911

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D12912