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
if not execution_date:
            flash('Invalid execution date', 'error')
            return redirect(origin)
        execution_date = timezone.parse(execution_date)
        dag = current_app.dag_bag.get_dag(dag_id)
        if not dag:
            flash(f'Cannot find DAG: {dag_id}', 'error')
            return redirect(origin)
        new_dag_state = set_dag_run_state_to_success(dag, execution_date, commit=confirmed)
        if confirmed:
            flash(f'Marked success on {len(new_dag_state)} task instances')
            return redirect(origin)
        else:
            details = '\n'.join([str(t) for t in new_dag_state])
            response = self.render_template(
                'airflow/confirm.html',
                message="Here's the list of task instances you are about to mark as success",
                details=details,
            )
            return response
    @expose('/dagrun_failed', methods=['POST'])
    @auth.has_access(
        [
            (permissions.ACTION_CAN_READ, permissions.RESOURCE_DAG),
            (permissions.ACTION_CAN_EDIT, permissions.RESOURCE_DAG_RUN),
        ]
    )
    @action_logging
    def dagrun_failed(self):