Add an error if a forRoot method is used not in a root module

[ddubrava]

🚀 Feature Request

✍️ Description

forRoot must be used only in a root module, check this article - https://angular.io/guide/singleton-services#the-forroot-pattern. So we should add an error if it's imported in children modules.

[ddubrava]

We can't add an error, it seems it's not possible to determine when forRoot is used outside of a root module. We can only determine when it's used twice but it's not the solution.

Add some documentation changes here - 4e1a082