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

_No response_

### Additional information

_No response_

### Reproduction

Not needed

Beginne mit der Dokumentation zu Polymorphic relation unter https://loopback.io/doc/en/lb4/Polymorphic-relation.html und vergleiche deren Delivery-Beispiel mit den im Issue gemeldeten Feldern. Aktualisiere das Beispiel so, dass die Relation deliverableId und deliverableType verwendet, und überprüfe anschließend, dass das dokumentierte Beispiel nicht mehr das nicht vorhandene Feld deliverable anfordert.

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