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
return redirect(self.get_redirect())
        else:
            suc_count = fail_count = 0
            for k, v in variable_dict.items():
                try:
                    models.Variable.set(k, v, serialize_json=not isinstance(v, str))
                except Exception as e:  
                    logging.info('Variable import failed: %s', repr(e))
                    fail_count += 1
                else:
                    suc_count += 1
            flash(f"{suc_count} variable(s) successfully updated.")
            if fail_count:
                flash(f"{fail_count} variable(s) failed to be updated.", 'error')
            self.update_redirect()
            return redirect(self.get_redirect())
class JobModelView(AirflowModelView):
    """View to show records from Job table"""
    route_base = '/job'
    datamodel = AirflowModelView.CustomSQLAInterface(BaseJob)  
    class_permission_name = permissions.RESOURCE_JOB
    method_permission_name = {
        'list': 'read',
    }
    base_permissions = [
        permissions.ACTION_CAN_READ,
        permissions.ACTION_CAN_ACCESS_MENU,
    ]
    list_columns = [
        'id',