Refactor comments in feature_nulldummy.py
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

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