[specification] Mention type inference for redirecting factories

Cf. https://github.com/dart-lang/sdk/issues/48796. The spec does not mention that type inference takes place in cases like this one, and this is a real source of confusion because it could also use i2b:

```dart
class C {
factory C() = D; // Means `= D`, not `= D`.
}

class D implements C {}
```

Read the specification sections covering redirecting factories and type inference, then compare the example with dart-lang/sdk issue 48796. Done means the specification explicitly states how type inference applies in this case and resolves the possible i2b interpretation.

Written by the indexing model from the issue text.