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
highlight(
                    config,
                    lexers.IniLexer(),  
                    HtmlFormatter(noclasses=True),
                )
            )
            return self.render_template(
                'airflow/config.html',
                pre_subtitle=settings.HEADER + "  v" + airflow.__version__,
                code_html=code_html,
                title=title,
                subtitle=subtitle,
                table=table,
            )
class RedocView(AirflowBaseView):
    """Redoc Open API documentation"""
    default_view = 'redoc'
    @expose('/redoc')
    def redoc(self):
        """Redoc API documentation."""
        openapi_spec_url = url_for("/api/v1./api/v1_openapi_yaml")
        return self.render_template('airflow/redoc.html', openapi_spec_url=openapi_spec_url)
class DagFilter(BaseFilter):
    """Filter using DagIDs"""
    def apply(self, query, func):  
        if current_app.appbuilder.sm.has_all_dags_access():
            return query
        filter_dag_ids = current_app.appbuilder.sm.get_accessible_dag_ids(g.user)
        return query.filter(self.model.dag_id.in_(filter_dag_ids))
class AirflowModelView(ModelView):