Add "refs" property to "Actions" in API documentation

I have spent a lot of time trying to figure out how to call non static functions from my components via the navigation bar buttons rendered in App.js (router file). I eventually found out all you need to do is simply use `Actions.refs.sceneKey.functionName()` after coming across this issue thread (https://github.com/aksonov/react-native-router-flux/issues/2298).

Could save other people a lot of time if this is added to the documentation.