Update circular-dependency.md

Applied review

Author: jakobwgnr

content/fundamentals/circular-dependency.md

@@ -6,7 +6,7 @@ While circular dependencies should be avoided where possible, you can't always d
 
 We also describe resolving circular dependencies between modules.
 
-> warning **Warning** A circular dependency might also be caused when using "barrel files"/index.ts files to group imports. Barrel files should be omitted when it comes to modules/providers. For more details please also see [this github issue](https://github.com/nestjs/nest/issues/1181#issuecomment-430197191). 
+> warning **Warning** A circular dependency might also be caused when using "barrel files"/index.ts files to group imports. Barrel files should be omitted when it comes to modules/providers. So barrel files should not be used when importing files within the same directory as the barrel file, i.e. `cats/cats.controller` should not import `cats` to import the `cats/cats.service` file. For more details please also see [this github issue](https://github.com/nestjs/nest/issues/1181#issuecomment-430197191). 
 
 #### Forward reference