Explicitly state what happens if `order_by` is called more than once. (#8791)

Explicitly state what happens if `order_by` is called more than once. (#8791)

* Explicitly state what happens if `order_by` is called more than once.

The existing docs cover how to clear existing `order_by` clauses but don't actually describe the behavior of calling `order_by` multiple times with different clauses.

* Also update Select.order_by.