Add documentation on how to display python deprecation warnings

Summary:
This is a follow-up of D3462.
It is a good idea to check against the use of deprecated features on
regular basis, but doing so in python is non-trivial. This diff adds a
chapter in the functional test documentation explaining how to display
the python deprecation warnings.

Test Plan: Read it.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3470