Improve docstrings and add type hints

[rettinghaus]

This PR is another step to close #21.

[bertsky]

Good work, much appreciated!

Just 2 caveats... would you like me to address these, or rather fix them yourself?

[bertsky]

Thx!

[bertsky]

I would like to rebase so we get a cleaner history from this:

• fd89a23 as fixup
• 5cca001 as fixup
• f628a0c as fixup
• 57b0c0d as fixup

Naturally, this would involve a force-push afterwards. Can I do it myself on your branch, or in a new PR, or would you kindly do that, too?