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
"""Set state to success."""
        try:
            count = 0
            altered_tis = []
            for dr in (
                session.query(DagRun).filter(DagRun.id.in_([dagrun.id for dagrun in drs])).all()
            ):  
                count += 1
                altered_tis += set_dag_run_state_to_success(
                    current_app.dag_bag.get_dag(dr.dag_id), dr.execution_date, commit=True, session=session
                )
            altered_ti_count = len(altered_tis)
            flash(
                "{count} dag runs and {altered_ti_count} task instances "
                "were set to success".format(count=count, altered_ti_count=altered_ti_count)
            )
        except Exception:  
            flash('Failed to set state', 'error')
        return redirect(self.get_default_url())
    @action('clear', "Clear the state", "All task instances would be cleared, are you sure?", single=False)
    @provide_session
    def action_clear(self, drs, session=None):
        """Clears the state."""
        try:
            count = 0
            cleared_ti_count = 0
            dag_to_tis = {}
            for dr in session.query(DagRun).filter(DagRun.id.in_([dagrun.id for dagrun in drs])).all():