When writing docstrings, we need to focus on the essential information that our audience needs. When we look at using a library module, what do we need to know? Whatever questions we ask, other programmers will often have similar questions. There are two boundaries that we should stay inside when we write docstrings:
It's best to avoid abstract overviews, high-level requirements, user stories, or background that is not tied directly to the code. We should focus the docstring on the code itself. We should provide the background in a separate document. A tool such as Sphinx can combine background material and code into a single document.
It's best to also avoid overly detailed how it works implementation trivia. The code is readily available, so there's no point in recapitulating the code in the documentation. If the code is too obscure, perhaps it should be rewritten to make it clearer.
Perhaps the single most important thing that developers want is a working example...