setRouteLeaveHook not documented in API.md

`setRouteLeaveHook` is found here [docs/guides/ConfirmingNavigation](https://github.com/reactjs/react-router/blob/master/docs/guides/ConfirmingNavigation.md)

But I couldn't found this method documented in [docs/API](https://github.com/reactjs/react-router/blob/master/docs/API.md).

Create a PR?
