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
"""Clears the action."""
        try:
            dag_to_tis = {}
            for ti in task_instances:
                dag = current_app.dag_bag.get_dag(ti.dag_id)
                task_instances_to_clean = dag_to_tis.setdefault(dag, [])
                task_instances_to_clean.append(ti)
            for dag, task_instances_list in dag_to_tis.items():
                models.clear_task_instances(task_instances_list, session, dag=dag)
            session.commit()
            flash(f"{len(task_instances)} task instances have been cleared")
            self.update_redirect()
            return redirect(self.get_redirect())
        except Exception as e:  
            flash(f'Failed to clear task instances: "{e}"', 'error')
            return None
    @provide_session
    def set_task_instance_state(self, tis, target_state, session=None):