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
"""Multiple delete action."""
        self.datamodel.delete_all(items)
        self.update_redirect()
        return redirect(self.get_redirect())
    def pre_add(self, item):
        """Pre add hook."""
        item.execution_date = timezone.make_aware(item.execution_date)
        item.value = XCom.serialize_value(item.value)
    def pre_update(self, item):