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
'dag_id': wwwutils.dag_link,
    }
class TaskRescheduleModelView(AirflowModelView):
    """View to show records from Task Reschedule table"""
    route_base = '/taskreschedule'
    datamodel = AirflowModelView.CustomSQLAInterface(models.TaskReschedule)  
    related_views = [DagRunModelView]
    class_permission_name = permissions.RESOURCE_TASK_RESCHEDULE
    method_permission_name = {
        'list': 'read',
    }
    base_permissions = [
        permissions.ACTION_CAN_READ,
        permissions.ACTION_CAN_ACCESS_MENU,
    ]
    list_columns = [
        'id',
        'dag_id',
        'task_id',
        'execution_date',
        'try_number',
        'start_date',
        'end_date',
        'duration',
        'reschedule_date',
    ]
    search_columns = ['dag_id', 'task_id', 'execution_date', 'start_date', 'end_date', 'reschedule_date']
    base_order = ('id', 'desc')
    base_filters = [['dag_id', DagFilter, lambda: []]]
    def duration_f(self):