docs: Everything needs doctestable examples

See [`dffml/util/data.py`](https://github.com/intel/dffml/blob/1ef131420bfed7ec049271a26f2ac577ed92b8c8/dffml/util/data.py) for an example.

We need to follow the [Numpy Style](https://intel.github.io/dffml/master/contributing/style.html#docstrings) too.