docs: Add more useful example for KeysSubset

Piece of code to think:

```python
def mapping_region(order: Order) -> Dict[str, Any]:
    region = {
        'id': order.get('region_id'),
        'name': order.get('region_name')
    }

    return {'region': region}


KeysSubset('region_id', 'region_name'): mapping_region,
```