Inform the user about the fact that query builders implement `Promise`

I had opened [a discussion](https://github.com/drizzle-team/drizzle-orm/discussions/1637) in the ORM repo about adding a feature that transforms the query's result after it's been obtained from the driver, but I had forgotten that this is already possible using the `then` method on the query builder, since it just extends `Promise`.

I feel like the fact that the query builders extend `Promise` is pretty easy to miss, and could be something worth noting in the documentation.

A frequently asked feature in the core API is some `findFirst` and `findFirstOrThrow` equivalent, and the documentation can provide an example as to how a developer than implement such functionality:

```ts
const user = db.select().from(users).limit(1).then((result) => result[0]);
```