Polymorphic Relation - Wrong Doc

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### Describe the bug

Hi there!

Let's suppose a base Delivery model with 3 fields: id, deliverableType, and deliverableId.
If you follow the [docs](https://loopback.io/doc/en/lb4/Polymorphic-relation.html), you'll get this error when requesting some deliverable relations:

> Request GET /deliveries?filter=whatever failed with status code 500. Error: ER_BAD_FIELD_ERROR: Unknown column 'deliverable' in 'field list'

This is caused by this part of code, as you're asking it to search on the corresponding datasource for the field "deliverable" which doesn't exist:

```
class Delivery extends Entity {
@hasOne(() => Deliverable, {polymorphic: true})
deliverable: Deliverable;

deliverableType: string;
}
```

Instead, it should be like this (search deliverableId on deliverableType's model):
```
class Delivery extends Entity {
@hasOne(() => Deliverable, {polymorphic: true})
deliverableId: string;

deliverableType: string;
id: string;
}
```

With that change, polymorphic relation is working as expected (assuming repositories are correctly configured)

### Logs

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### Additional information

_No response_

### Reproduction

Not needed

Comienza con la documentación de Polymorphic relation en https://loopback.io/doc/en/lb4/Polymorphic-relation.html y compara su ejemplo de Delivery con los campos indicados en el issue. Actualiza el ejemplo para que la relación use deliverableId y deliverableType, y verifica después que el ejemplo documentado ya no solicite el campo inexistente deliverable.

Escrito por el modelo de indexación a partir del texto del issue.