[doc] sample code for test framework p2p objects

Summary:
This is a backport of core#19804 [2/4]
https://github.com/bitcoin/bitcoin/pull/19804/commits/7a0de46aeafb351cffa3410e1aae9809fd4698ad

Test Plan: Proofreading

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10322