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

Inizia dalla documentazione di Polymorphic relation all’indirizzo https://loopback.io/doc/en/lb4/Polymorphic-relation.html e confronta il relativo esempio Delivery con i campi segnalati nell’issue. Aggiorna l’esempio in modo che la relazione utilizzi deliverableId e deliverableType, quindi verifica che l’esempio documentato non richieda più il campo inesistente deliverable.

Scritto dal modello di indicizzazione a partire dal testo della issue.