title: what is the context argument in BaseOperator.xcom_pull
body: <p>I was reading the API Docs, and was unclear what the <em>context</em> argument is for <a href="https://pythonhosted.org/airflow/code.html?highlight=xcom#airflow.models.BaseOperator.xcom_pull" rel="noreferrer">BaseOperator.xcom_pull</a>.</p>
  
  <p>I thought it would be <strong>dag.default_args</strong>, but I receive <code>KeyError: 'ti'</code></p>
  
  <p>I performed an xcom_push within a prior task following the push() example <a href="https://github.com/apache/incubator-airflow/blob/master/airflow/example_dags/example_xcom.py" rel="noreferrer">here</a>.</p>