[linting] Require docstrings for all (public) methods

All public API functions should be documented. To enforce this requirement, we should add a linter check that requires any newly-added functions to contain a docstring.

We should discuss whether it makes more sense to enforce docstrings for all functions or only public API functions. Having docstrings for private functions could help us remember what our internal functions are used for, but maybe we don't need to enforce this as a strict requirement.