Docs are misleading for `casing`

When the `casing` parameter is described [in the docs](https://orm.drizzle.team/docs/sql-schema-declaration#camel-and-snake-casing), we see this example:

```ts
// schema.ts
import { drizzle } from "drizzle-orm/node-postgres";
import { integer, pgTable, varchar } from "drizzle-orm/pg-core";
export const users = pgTable('users', {
id: integer(),
firstName: varchar()
})

// db.ts
const db = drizzle({ connection: process.env.DATABASE_URL, casing: 'snake_case' })

// query.ts
await db.select().from(users);
```

However, this clearly won't work. The `casing` param is provided at runtime, and drizzle is not aware of any column aliasing settings during migration. As a minimum working example, this fails. The column is named `firstName` in the database.

---

Now, through some amount of experimentation, I discovered that I can provide a `casing` option to `defineConfig` like this:

```ts
export default defineConfig({
// ...
casing: 'snake_case'
});
```

But, as far as I can tell, this is [undocumented](https://orm.drizzle.team/docs/drizzle-config-file).

---

Proposal: if I'm correct in the above, I propose to properly document the `casing` option to `defineConfig`, and also make it clear that this option must be specified when discussing the `casing` in the schema declaration docs.

I am happy to make a PR, but I need someone more knowledgeable in Drizzle to confirm that what I've said is accurate.