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+.

阅读 docs/version-7-upgrade.md，并将其中关于 USE_EMULATOR 的指导与 docs/emulators/emulators.md“6.1 Method.”下的示例进行比较。更新 v7+ 的 Emulators 指南，使文档中的 USE_AUTH_EMULATOR 值与 breaking change 格式一致，然后验证示例及其周围的说明是否一致。

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