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 statistics."""
        dr = models.DagRun
        allowed_dag_ids = current_app.appbuilder.sm.get_accessible_dag_ids(g.user)
        dag_state_stats = session.query(dr.dag_id, dr.state, sqla.func.count(dr.state)).group_by(
            dr.dag_id, dr.state
        )