[doc] package deduplication

Summary:
Clarify that packages transactions that are already in the mempool are removed from the package.
This is a backport of core#24310 (only the last commit is relevant https://github.com/bitcoin/bitcoin/pull/24310/commits/77202f0554dcbbbb167d0ed3927cca0bf4609ce8)

Test Plan: Read it.

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D16353