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+.

Lee docs/version-7-upgrade.md y compara sus indicaciones sobre USE_EMULATOR con el ejemplo de docs/emulators/emulators.md en “6.1 Method.” Actualiza la guía de Emulators para v7+ de modo que el valor documentado de USE_AUTH_EMULATOR coincida con el formato del breaking change y, después, verifica que el ejemplo y la explicación circundante sean coherentes.

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