docs(fundamentals): add mention on limitation of auto mocking

Author: micalevisk

content/fundamentals/unit-testing.md

@@ -194,9 +194,11 @@ describe('CatsController', () => {
 });
 ```
 
+You can also retrieve these mocks out of the testing container as you normally would custom providers, `moduleRef.get(CatsService)`.
+
 > info **Hint** A general mock factory, like `createMock` from [`@golevelup/ts-jest`](https://github.com/golevelup/nestjs/tree/master/packages/testing) can also be passed directly.
 
-You can also retrieve these mocks out of the testing container as you normally would custom providers, `moduleRef.get(CatsService)`.
+> info **Hint** `REQUEST` and `INQUIRER` providers cannot be auto-mocked because they're already pre-defined in the context. However, they can be _overwritten_ using the custom provider syntax or by utilizing the `.overrideProvider` method.
 
 #### End-to-end testing