Add missing docstrings

[approxit]

As #1086 introduced unified scripts in task runners, flake8 is rising additional errors that were not looked before. Errors D100, D101, D102, D103, D106 were marked as ignored, to keep #1086 as compact as possible, but its not ideal.

Mentioned errors should be removed from ignore list, and raised problems with missing docs should be prepared to satisfy flake8.