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
"""Get Dag details."""
        dag_id = request.args.get('dag_id')
        dag = current_app.dag_bag.get_dag(dag_id)
        title = "DAG Details"
        root = request.args.get('root', '')
        states = (
            session.query(TaskInstance.state, sqla.func.count(TaskInstance.dag_id))
            .filter(TaskInstance.dag_id == dag_id)
            .group_by(TaskInstance.state)
            .all()
        )
        active_runs = models.DagRun.find(dag_id=dag_id, state=State.RUNNING, external_trigger=False)
        tags = session.query(models.DagTag).filter(models.DagTag.dag_id == dag_id).all()
        return self.render_template(
            'airflow/dag_details.html',
            dag=dag,
            title=title,
            root=root,
            states=states,
            State=State,
            active_runs=active_runs,
            tags=tags,
        )