add text column to explain what the variable is used for

same as https://github.com/apache/airflow/issues/10840 just for Variables.


airflow/www/views.py
last_task_instance_query_result, running_task_instance_query_result
            ).alias('final_ti')
        else:
            final_task_instance_query_result = running_task_instance_query_result.subquery('final_ti')
        qry = session.query(
            final_task_instance_query_result.c.dag_id,
            final_task_instance_query_result.c.state,
            sqla.func.count(),
        ).group_by(final_task_instance_query_result.c.dag_id, final_task_instance_query_result.c.state)
        data = {}
        for dag_id, state, count in qry:
            if dag_id not in data:
                data[dag_id] = {}
            data[dag_id][state] = count
        payload = {}
        for dag_id in filter_dag_ids:
            payload[dag_id] = []
            for state in State.task_states:
                count = data.get(dag_id, {}).get(state, 0)
                payload[dag_id].append({'state': state, 'count': count})
        return wwwutils.json_response(payload)
    @expose('/last_dagruns', methods=['POST'])
    @auth.has_access(
        [
            (permissions.ACTION_CAN_READ, permissions.RESOURCE_DAG),
            (permissions.ACTION_CAN_READ, permissions.RESOURCE_DAG_RUN),
        ]
    )
    @provide_session
    def last_dagruns(self, session=None):