Add docstrings

Include docstrings at some point to make clear what each function/method is doing to reviewers/contributors/users: https://www.python.org/dev/peps/pep-0257/

The issue names no files, tests, or entry points; start by surveying the repository's Python functions and methods and read PEP 257. Define the documentation scope before editing, then verify that the agreed functions and methods have clear docstrings suitable for reviewers, contributors, and users.

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