Audit: write up where we need to add docstring runnable examples

We should add examples to the doc strings of as many functions as possible.

We can also set these up as doctests, so that when we build the docs, the output of the method call is tested against the example.

We do this in featuretools. For example:
https://docs.featuretools.com/en/stable/generated/featuretools.primitives.Count.html#featuretools.primitives.Count