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
"""
        Sets a value for an Airflow Variable with a given Key
        :param key: Variable Key
        :param value: Value to set for the Variable
        :param serialize_json: Serialize the value to a JSON string
        :param session: SQL Alchemy Sessions
        """
        env_var_name = "AIRFLOW_VAR_" + key.upper()
        if env_var_name in os.environ:
            log.warning(
                "You have the environment variable %s defined, which takes precedence over reading "
                "from the database. The value will be saved, but to read it you have to delete "
                "the environment variable.",
                env_var_name,
            )
        if serialize_json:
            stored_value = json.dumps(value, indent=2)
        else:
            stored_value = str(value)
        Variable.delete(key, session=session)
        session.add(Variable(key=key, val=stored_value))
        session.flush()
    @classmethod
    @provide_session
    def delete(cls, key: str, session: Session = None) -> int: