This annotation is not supposed to be used by Mockito end-users. Instead, we
use it to annotate methods for Static Analysis tools, including FindBugs and ErrorProne.
These tools can check whether the return value of our Mockito methods are actually
used. As such, Mockito State Validation can be performed at compile-time rather than run-time.
This annotation is public, because we have to use it in multiple packages.