[net processing] Improve documentation for Peer destruction/locking

Summary:
As per title, only comments.

Partial backport of core#19829:
https://github.com/bitcoin/bitcoin/pull/19829/commits/717a374e74b64b7b90bc1b2995e8900212bd0bfe

Ref T1696.

Test Plan: Read the comments.

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Maniphest Tasks: T1696

Differential Revision: https://reviews.bitcoinabc.org/D10865