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
"""
        Delete an Airflow Variable for a given key
        :param key: Variable Key
        :param session: SQL Alchemy Sessions
        """
        return session.query(cls).filter(cls.key == key).delete()
    def rotate_fernet_key(self):
        """Rotate Fernet Key"""
        fernet = get_fernet()
        if self._val and self.is_encrypted:
            self._val = fernet.rotate(self._val.encode('utf-8')).decode()
    @staticmethod
    def get_variable_from_secrets(key: str) -> Optional[str]:
        """
        Get Airflow Variable by iterating over all Secret Backends.
        :param key: Variable Key
        :return: Variable Value
        """
        for secrets_backend in ensure_secrets_loaded():
            var_val = secrets_backend.get_variable(key=key)
            if var_val is not None:
                return var_val
        return None