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
from collections import defaultdict
from datetime import datetime, timedelta
from json import JSONDecodeError
from operator import itemgetter
from typing import Dict, List, Optional, Tuple
from urllib.parse import parse_qsl, unquote, urlencode, urlparse
import lazy_object_proxy
import nvd3
import sqlalchemy as sqla
from flask import (
    Markup,
    Response,
    abort,
    before_render_template,
    current_app,
    escape,
    flash,
    g,
    jsonify,
    make_response,
    redirect,
    render_template,
    request,
    session as flask_session,
    url_for,
)
from flask_appbuilder import BaseView, ModelView, expose
from flask_appbuilder.actions import action
from flask_appbuilder.fieldwidgets import Select2Widget
from flask_appbuilder.models.sqla.filters import BaseFilter