Refactor comments in feature_nulldummy.py

Summary: As per title

Test Plan:

./test/functional/test_runner.py feature_nulldummy

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: dagurval, teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2444