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+.

閱讀 docs/version-7-upgrade.md，並將其中關於 USE_EMULATOR 的指引與 docs/emulators/emulators.md「6.1 Method.」下的範例進行比較。更新 v7+ 的 Emulators 指南，使文件中的 USE_AUTH_EMULATOR 值與 breaking change 格式一致，然後確認範例及其周圍的說明是否一致。

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