add text column to explain what the variable is used for

same as https://github.com/apache/airflow/issues/10840 just for Variables.


airflow/models/variable.py
#
#
#
import json
import logging
import os
from typing import Any, Optional
from cryptography.fernet import InvalidToken as InvalidFernetToken
from sqlalchemy import Boolean, Column, Integer, String, Text
from sqlalchemy.ext.declarative import declared_attr
from sqlalchemy.orm import Session, synonym
from airflow.configuration import ensure_secrets_loaded
from airflow.models.base import ID_LEN, Base
from airflow.models.crypto import get_fernet
from airflow.utils.log.logging_mixin import LoggingMixin
from airflow.utils.session import provide_session
log = logging.getLogger()
class Variable(Base, LoggingMixin):
    """
    Variables are a generic way to store and retrieve arbitrary content or settings
    as a simple key value store within Airflow.
    """
    __tablename__ = "variable"
    __NO_DEFAULT_SENTINEL = object()
    id = Column(Integer, primary_key=True)
    key = Column(String(ID_LEN), unique=True)
    _val = Column('val', Text)
    is_encrypted = Column(Boolean, unique=False, default=False)
    def __init__(self, key=None, val=None):