useClass is missing when importing the module via forRootAsync

Hi,

As NestJS's dynamic module support using a class instead of only using a factory method, why is `useClass` missing in the module? Was it deliberately removed? or Was it never implemented?
https://docs.nestjs.com/fundamentals/custom-providers#class-providers-useclass

useClass keeps your main module file free of all details and help for clear modularization. 