[AIRFLOW-575] Clarify tutorial and FAQ about schedule_interval always inheriting from DAG object

[lauralorenz]

Dear Airflow Maintainers,

Please accept the following PR that

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

[lauralorenz]

FYI, I couldn't figure out how to change the API reference for BaseSensorOperator to not specify schedule_interval as a parameter on the function signature, since as far as I can tell it isn't in the docstring so I don't know how it is still there. But that might be good to take out too for this point.

[landscape-bot]

Repository health decreased by 1% when pulling a4f8e50 on lauralorenz:schedule_interval_default_args_docs into aecfa7c on airbnb:master.

• 148 new problems were found (including 23 errors and 100 code smells).
• 737 problems were fixed (including 6 errors and 82 code smells).

[coveralls]

Coverage increased (+0.03%) to 67.074% when pulling a4f8e506c725fd50f6c12da0f44979a20476ff8b on lauralorenz:schedule_interval_default_args_docs into aecfa7c on airbnb:master.

[artwr]

@lauralorenz This looks good to me, sorry it took us a long time to get to this. I created a JIRA issue for this. https://issues.apache.org/jira/browse/AIRFLOW-575 If you can just have your PR reference this, I will merge it into master.

[lauralorenz]

Woo first real git rebase experience. I rebased with upstream master and amended my commit message to include the JIRA reference. Same with the PR title. I think this is good now, let me know if it needs anything else. @artwr

[artwr]

@lauralorenz Thank you! This has been merged to master.

[lauralorenz]

Cool, do I need to do anything to the JIRA issue too?

[artwr]

Just marked it as resolved. Thanks again.