future<shared_future<T>>::unwrap does not exist

The docs say:

```
Member function unwrap() EXTENSION
template
future future< future>::unwrap(); // EXTENSION
template
boost::shared_future future< boost::shared_future>::unwrap(); // EXTENSION
```

[thread.hpp:1887](https://github.com/boostorg/thread/blob/409c98f8b745e72bc326e93bfaf8a353d94a69b0/include/boost/thread/future.hpp#L1887) appears to define the future>::unwrap, but it doesn't appear that there's a specialization for future> to provide the documented `unwrap` method.