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."""
        self.datamodel.delete_all(items)
        self.update_redirect()
        return redirect(self.get_redirect())
    @action('set_running', "Set state to 'running'", '', single=False)
    @provide_session
    def action_set_running(self, drs, session=None):
        """Set state to running."""
        try:
            count = 0
            for dr in (
                session.query(DagRun).filter(DagRun.id.in_([dagrun.id for dagrun in drs])).all()
            ):  
                count += 1
                dr.start_date = timezone.utcnow()
                dr.state = State.RUNNING
            session.commit()
            flash(f"{count} dag runs were set to running")
        except Exception as ex:  
            flash(str(ex), 'error')
            flash('Failed to set state', 'error')
        return redirect(self.get_default_url())
    @action(
        'set_failed',
        "Set state to 'failed'",
        "All running task instances would also be marked as failed, are you sure?",
        single=False,
    )
    @provide_session
    def action_set_failed(self, drs, session=None):