[AIRFLOW-575] Clarify tutorial and FAQ about schedule_interval alwa…

…ys inheriting from DAG object

- Update the tutorial with a comment helping to explain the use of default_args and
include all the possible parameters in line
- Clarify in the FAQ the possibility of an unexpected default `schedule_interval`in case
airflow users mistakenly try to overwrite the default `schedule_interval` in a DAG's
`default_args` parameter

airflow/example_dags/tutorial.py

@@ -10,6 +10,8 @@
 seven_days_ago = datetime.combine(datetime.today() - timedelta(7),
                                   datetime.min.time())
 
+# these args will get passed on to each operator
+# you can override them on a per-task basis during operator initialization
 default_args = {
     'owner': 'airflow',
     'depends_on_past': False,
@@ -22,11 +24,19 @@
     # 'queue': 'bash_queue',
     # 'pool': 'backfill',
     # 'priority_weight': 10,
-    # 'schedule_interval': timedelta(1),
     # 'end_date': datetime(2016, 1, 1),
+    # 'wait_for_downstream': False,
+    # 'dag': dag,
+    # 'adhoc':False,
+    # 'sla': timedelta(hours=2),
+    # 'execution_timeout': timedelta(seconds=300),
+    # 'on_failure_callback': some_function,
+    # 'on_success_callback': some_other_function,
+    # 'on_retry_callback': another_function,
+    # 'trigger_rule': u'all_success'
 }
 
-dag = DAG('tutorial', default_args=default_args)
+dag = DAG('tutorial', default_args=default_args, schedule_interval=timedelta(days=1))
 
 # t1, t2 and t3 are examples of tasks created by instantiating operators
 t1 = BashOperator(

docs/faq.rst

@@ -17,6 +17,11 @@ Here are some of the common causes:
 - Is your ``start_date`` set properly? The Airflow scheduler triggers the
   task soon after the ``start_date + scheduler_interval`` is passed.
 
+- Is your ``schedule_interval`` set properly? The default ``schedule_interval``
+  is one day (``datetime.timedelta(1)``). You must specify a different ``schedule_interval``
+  directly to the DAG object you instantiate, not as a ``default_param``, as task instances
+  do not override their parent DAG's ``schedule_interval``.
+
 - Is your ``start_date`` beyond where you can see it in the UI? If you
   set your it to some time say 3 months ago, you won't be able to see
   it in the main view in the UI, but you should be able to see it in the