Add documentation for test_shell submodule

Summary:
Documentation-only change.

This is a backport of Core PR17288 [7/7]
https://github.com/bitcoin/bitcoin/pull/17288/commits/19139ee034d20ebab1b91d3ac13a8eee70b59374

It includes a fix from PR17378:
https://github.com/bitcoin/bitcoin/pull/17378/commits/9c7806e4bf113bee6c32cff7b46493fd1a5aa0ba

Depends on D8246

Test Plan: Proof-reading

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8247