[core#19724 4/9][doc] Explain address handling logic in process messages

Summary:
Co-authored-by: Suhas Daftuar <sdaftuar@gmail.com>
This is a backport of core#19724 [4/9]
https://github.com/bitcoin/bitcoin/pull/19724/commits/ff6b9081add3a40d53b1cc1352b57eeb46e41d45

Depends on D9772

Test Plan: Proof-reading new comments

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9773