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
"""Formats log URL."""
        log_url = self.get('log_url')  
        return Markup(  
            '<a href="{log_url}"><span class="material-icons" aria-hidden="true">reorder</span></a>'
        ).format(log_url=log_url)
    def duration_f(self):