Method order_by_desc

src/query/helper.rs:645–652

Add an order_by expression (descending) ``` use sea_orm::{entity::*, query::*, tests_cfg::cake, DbBackend}; assert_eq!( cake::Entity::find() .order_by_desc(cake::Column::Id) .build(DbBackend::MySql) .to_string(), "SELECT `cake`.`id`, `cake`.`name` FROM `cake` ORDER BY `cake`.`id` DESC" ); ```

(mut self, col: C)

    /// );
    /// ```
    fn order_by_desc<C>(mut self, col: C) -> Self
    where
        C: IntoSimpleExpr,
    {
        self.query()
            .order_by_expr(col.into_simple_expr(), Order::Desc);
        self
    }

    /// Add an order_by expression with nulls ordering option
    /// ```