Added more explicit Tracer explanation
Added more explicit Tracer explanation

without this change it could be nonclear where Tracer comes from and how you can use it.

With this change hopefully it get properly explained

fixes #402

spring-cloud/spring-cloud-sleuth,spring-cloud/spring-cloud-sleuth,marcingrzejszczak/spring-cloud-sleuth,spring-cloud/spring-cloud-sleuth,marcingrzejszczak/spring-cloud-sleuth,spring-cloud/spring-cloud-sleuth,marcingrzejszczak/spring-cloud-sleuth,marcingrzejszczak/spring-cloud-sleuth,spring-cloud/spring-cloud-sleuth