Docs: Emulator guide missing breaking changes in >7.0

### How to reproduce these conditions

The [upgrade guide](https://github.com/angular/angularfire/blob/master/docs/version-7-upgrade.md) for v7 mentions a breaking change: "`USE_EMULATOR` DI token is now in the form of `['http://localhost:9099']`". This change is not documented in [the Emulators guide](https://github.com/angular/angularfire/blob/master/docs/emulators/emulators.md). The Emulators guide still shows:
`{ provide: USE_AUTH_EMULATOR, useValue: environment.useEmulators ? ['localhost', 9099] : undefined },`

This is shown under '6.1 Method'. Something should probably be written for v7+.

Leggi docs/version-7-upgrade.md e confronta le indicazioni relative a USE_EMULATOR con l’esempio in docs/emulators/emulators.md nella sezione “6.1 Method.” Aggiorna la guida Emulators per v7+ in modo che il valore documentato di USE_AUTH_EMULATOR corrisponda al formato della breaking change, quindi verifica che l’esempio e la spiegazione circostante siano coerenti.

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