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
"""Retrieve XCOM."""
        dag_id = request.args.get('dag_id')
        task_id = request.args.get('task_id')
        
        
        execution_date = request.args.get('execution_date')
        dttm = timezone.parse(execution_date)
        form = DateTimeForm(data={'execution_date': dttm})
        root = request.args.get('root', '')
        dm_db = models.DagModel
        ti_db = models.TaskInstance
        dag = session.query(dm_db).filter(dm_db.dag_id == dag_id).first()
        ti = session.query(ti_db).filter(ti_db.dag_id == dag_id and ti_db.task_id == task_id).first()
        if not ti:
            flash(f"Task [{dag_id}.{task_id}] doesn't seem to exist at the moment", "error")