Add docstring to custom files and functions

[vmatyus]

Is your feature request related to a problem?
Currently, there are plenty of function without docstring. Before these lines the pylint is set disabled like: # pylint: disable=missing-function-docstring.
As a product of this, the generated doc has not so much content. Here is a snapshot:

Describe the solution you'd like
It would be good the add some description about the role of the functions and its parameters, additionally a little description about the module.

Here is a doc that can help in the syntax of the docstrings: https://sphinx-rtd-tutorial.readthedocs.io/en/latest/docstrings.html#the-sphinx-docstring-format
https://www.sphinx-doc.org/en/master/usage/restructuredtext/domains.html#cross-referencing-python-objects

Describe alternatives you've considered

Additional context

[ricellis]

Worth noting that none of that common module is intended to be user API so we should probably exlude them from the doc entirely or otherwise label it non-API.