[docs] update DagBag class docstring to include all params (apache#…

…38814)

* update docstring for DagBag class

* break long line

* fix space

Signed-off-by: kalyanr <kalyan.ben10@live.com>

---------

Signed-off-by: kalyanr <kalyan.ben10@live.com>

airflow/models/dagbag.py

@@ -89,11 +89,16 @@ class DagBag(LoggingMixin):
     :param dag_folder: the folder to scan to find DAGs
     :param include_examples: whether to include the examples that ship
         with airflow or not
+    :param safe_mode: when ``False``, scans all python modules for dags.
+        When ``True`` uses heuristics (files containing ``DAG`` and ``airflow`` strings)
+        to filter python modules to scan for dags.
     :param read_dags_from_db: Read DAGs from DB if ``True`` is passed.
         If ``False`` DAGs are read from python files.
+    :param store_serialized_dags: deprecated parameter, same effect as `read_dags_from_db`
     :param load_op_links: Should the extra operator link be loaded via plugins when
         de-serializing the DAG? This flag is set to False in Scheduler so that Extra Operator links
         are not loaded to not run User code in Scheduler.
+    :param collect_dags: when True, collects dags during class initialization.
     """
 
     def __init__(