docs: Improve examples of Interceptors with more implementation details

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In https://loopback.io/doc/en/lb4/Interceptors.html#example-interceptors
there are different examples of interceptors.

To understand the example implementations of these interceptors, let's
also show an example implementation of `what` they are intercepting.

For example:

```ts
/**
* A binding provider class to produce an interceptor that validates the
* `name` argument
*/
class NameValidator implements Provider {
constructor(@inject('valid-names') private validNames: string[]) {}

value() {
return this.intercept.bind(this);
}

// (doc improvement)
// This interceptor works as an authorizer for certain endpoints,
// which takes in the first argument as the name.
async intercept(
invocationCtx: InvocationContext,
next: () => ValueOrPromise,
) {
const name = invocationCtx.args[0];
if (!this.validNames.includes(name)) {
throw new Error(
`Name '${name}' is not on the list of '${this.validNames}`,
);
}
return next();
}
}
```
It is not clear what this interceptor is attached to. Is it a function, a class, a request?

How do we know that the `name` we need to validate is in args[0] of the invocation context?

Providing more implementation details would shed some light on how to use them.

_See [Reporting Issues](http://loopback.io/doc/en/contrib/Reporting-issues.html) for more tips on writing good issues_

## Acceptane Criteria

- [ ] Refine the docs to include more details described above.
- [ ] For the example snippet, it implies we need to add more details to `InvocationContext`'s properties, like `args`, see https://loopback.io/doc/en/lb4/apidocs.context.invocationcontext.html#properties

Inizia con la documentazione di Interceptors nell’esempio collegato e rivedi le proprietà collegate dell’API InvocationContext, in particolare args. Chiarisci a cosa è associato l’interceptor, come gli argomenti del metodo intercettato vengono mappati su args[0], e aggiungi i dettagli di implementazione richiesti; i criteri di accettazione dovrebbero essere soddisfatti quando l’esempio è comprensibile senza queste domande senza risposta.

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