Minor improvements to add_nodes

Summary:
This is a backport of core PR14805 and PR14951.
Since the first one add a feature and the second reverts it, the
remaining is a refactor-only commit that improves the docstring for
add_nodes() and cuts a long line to improve readability:
https://github.com/bitcoin/bitcoin/pull/14951/commits/fa4b8c90d3d13c7382357ee9f75df4f4974aef20

Depends on D4557.

Test Plan:

./test/functional/test_runner.py

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D4559