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
"""Mark DagRun failed."""
        dag_id = request.form.get('dag_id')
        execution_date = request.form.get('execution_date')
        confirmed = request.form.get('confirmed') == 'true'
        origin = get_safe_url(request.form.get('origin'))
        return self._mark_dagrun_state_as_failed(dag_id, execution_date, confirmed, origin)
    @expose('/dagrun_success', methods=['POST'])
    @auth.has_access(
        [
            (permissions.ACTION_CAN_READ, permissions.RESOURCE_DAG),
            (permissions.ACTION_CAN_EDIT, permissions.RESOURCE_DAG_RUN),
        ]
    )
    @action_logging
    def dagrun_success(self):
        """Mark DagRun success"""
        dag_id = request.form.get('dag_id')
        execution_date = request.form.get('execution_date')
        confirmed = request.form.get('confirmed') == 'true'
        origin = get_safe_url(request.form.get('origin'))
        return self._mark_dagrun_state_as_success(dag_id, execution_date, confirmed, origin)
    def _mark_task_instance_state(