Add docstrings to DAG class methods

[martingrayson]

Is there an environment variable/some property of a dag to know the actual dags location on the file system?

[mistercrunch]

Here:
https://github.com/airbnb/airflow/blob/master/airflow/models.py#L1576

I need to add docstrings to some of these methods so that they surface in the API docs...

Since it's a property you can just use mydag.filepath

[martingrayson]

Thanks 👍