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
"""Formats duration."""
        end_date = self.get('end_date')  
        duration = self.get('duration')  
        if end_date and duration:
            return timedelta(seconds=duration)
        return None
    formatters_columns = {
        'log_url': log_url_formatter,
        'task_id': wwwutils.task_instance_link,
        'hostname': wwwutils.nobr_f('hostname'),
        'state': wwwutils.state_f,
        'execution_date': wwwutils.datetime_f('execution_date'),
        'start_date': wwwutils.datetime_f('start_date'),
        'end_date': wwwutils.datetime_f('end_date'),
        'queued_dttm': wwwutils.datetime_f('queued_dttm'),
        'dag_id': wwwutils.dag_link,
        'duration': duration_f,
    }
    @provide_session
    @action(
        'clear',
        lazy_gettext('Clear'),
        lazy_gettext(
            'Are you sure you want to clear the state of the selected task'
            ' instance(s) and set their dagruns to the running state?'
        ),
        single=False,
    )
    def action_clear(self, task_instances, session=None):