message: docs: clarify that `forRoot` can be called in other modules
  The faq on `forRoot` implies that you cannot and should not call `forRoot` for an import
  outside the AppModule. This is technically incorrect. This commit clarifies
  the statement and refers the reader to the hierarchical injectors guide.
  fixes
  PR Close
diff: @@ -180,6 +180,12 @@ Only call and import a `forRoot()` result in the root application module, `AppMo
  Avoid importing it in any other module, particularly in a lazy-loaded module. For more
  information on `forRoot()` see [the `forRoot()` pattern](guide/singleton-services#the-forroot-pattern) section of the [Singleton Services](guide/singleton-services) guide.
  +<div class="alert is-helpful">
  +Note: the `forRoot()` import can be used in a module other than `AppModule`. Importantly,
  +`forRoot()` should only be called once, and the module that imports the `forRoot()` needs to be available to
  +the root `ModuleInjector`. For more information, refer to the guide on [Hierarchical injectors](guide/hierarchical-dependency-injection#moduleinjector).
  +</div>
  +
  For a service, instead of using `forRoot()`, specify `providedIn: 'root'` on the service's `@Injectable()` decorator, which
  makes the service automatically available to the whole application and thus singleton by default.
  