Clearly describe API of callback in code (#7279)
Clearly describe API of callback in code (#7279)

* Clearly describe API of callback in code

* fix syntax error
ReactTraining/react-router,reactjs/react-router,rackt/react-router,reactjs/react-router,ReactTraining/react-router,ReactTraining/react-router,rackt/react-router,ReactTraining/react-router