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+.

Read docs/version-7-upgrade.md and compare its USE_EMULATOR guidance with the example in docs/emulators/emulators.md under “6.1 Method.” Update the Emulators guide for v7+ so the documented USE_AUTH_EMULATOR value matches the breaking-change format, then verify the example and surrounding explanation are consistent.

Written by the indexing model from the issue text.