[AIRFLOW-2864] Fix docstrings for SubDagOperator (#3712)
[AIRFLOW-2864] Fix docstrings for SubDagOperator (#3712)

The docstrings are currently in the `__init__` method, due to which they are not automatically shown in the Sphinx documentation.
owlabs/incubator-airflow,owlabs/incubator-airflow,owlabs/incubator-airflow,owlabs/incubator-airflow