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
running_dag_run_query_result.c.dag_id == TaskInstance.dag_id,
                running_dag_run_query_result.c.execution_date == TaskInstance.execution_date,
            ),
        )
        
        if conf.getboolean('webserver', 'SHOW_RECENT_STATS_FOR_COMPLETED_RUNS', fallback=True):
            
            last_dag_run = (
                session.query(DagRun.dag_id, sqla.func.max(DagRun.execution_date).label('execution_date'))
                .join(DagModel, DagModel.dag_id == DagRun.dag_id)
                .filter(DagRun.state != State.RUNNING, DagModel.is_active)
                .group_by(DagRun.dag_id)
            )
            
            
            last_dag_run = last_dag_run.filter(DagRun.dag_id.in_(filter_dag_ids))
            last_dag_run = last_dag_run.subquery('last_dag_run')
            
            
            
            last_task_instance_query_result = session.query(
                TaskInstance.dag_id.label('dag_id'), TaskInstance.state.label('state')
            ).join(
                last_dag_run,
                and_(
                    last_dag_run.c.dag_id == TaskInstance.dag_id,
                    last_dag_run.c.execution_date == TaskInstance.execution_date,
                ),
            )
            final_task_instance_query_result = union_all(