Document what cases checkRedundantReturns covers

I've got `checkRedundantReturns` enabled. So far I've gotten a warning for `@return {undefined}` (which I understand) but then I just got it for:

```
/**
 * @return {UnauthorizedException} the exception
 */
function UnauthorizedException()
{
  ...
}
```

Why is the `@return` redundant? Isn't it possible for the constructor to return a subclass?
