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
"""Shows configuration."""
        raw = request.args.get('raw') == "true"
        title = "Airflow Configuration"
        subtitle = AIRFLOW_CONFIG
        
        if conf.getboolean("webserver", "expose_config"):
            with open(AIRFLOW_CONFIG) as file:
                config = file.read()
            table = [
                (section, key, value, source)
                for section, parameters in conf.as_dict(True, True).items()
                for key, (value, source) in parameters.items()
            ]
        else:
            config = (
                "# Your Airflow administrator chose not to expose the "
                "configuration, most likely for security reasons."
            )
            table = None
        if raw:
            return Response(response=config, status=200, mimetype="application/text")
        else:
            code_html = Markup(