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 slots rendering."""
        pool_id = self.get('pool')  
        running_slots = self.get('running_slots')  
        if pool_id is not None and running_slots is not None:
            url = url_for('TaskInstanceModelView.list', _flt_3_pool=pool_id, _flt_3_state='running')
            return Markup("<a href='{url}'>{running_slots}</a>").format(  
                url=url, running_slots=running_slots
            )
        else:
            return Markup('<span class="label label-danger">Invalid</span>')
    def fqueued_slots(self):
        """Queued slots rendering."""
        pool_id = self.get('pool')  
        queued_slots = self.get('queued_slots')  
        if pool_id is not None and queued_slots is not None:
            url = url_for('TaskInstanceModelView.list', _flt_3_pool=pool_id, _flt_3_state='queued')
            return Markup("<a href='{url}'>{queued_slots}</a>").format(  
                url=url, queued_slots=queued_slots
            )
        else:
            return Markup('<span class="label label-danger">Invalid</span>')
    formatters_columns = {'pool': pool_link, 'running_slots': frunning_slots, 'queued_slots': fqueued_slots}
    validators_columns = {'pool': [validators.DataRequired()], 'slots': [validators.NumberRange(min=-1)]}
class VariableModelView(AirflowModelView):