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
"""Mark task as failed."""
        dag_id = request.form.get('dag_id')
        task_id = request.form.get('task_id')
        origin = get_safe_url(request.form.get('origin'))
        execution_date = request.form.get('execution_date')
        confirmed = request.form.get('confirmed') == "true"
        upstream = request.form.get('failed_upstream') == "true"
        downstream = request.form.get('failed_downstream') == "true"
        future = request.form.get('failed_future') == "true"
        past = request.form.get('failed_past') == "true"
        return self._mark_task_instance_state(
            dag_id,
            task_id,
            origin,