Merge pull request #2423 from taion/API-onEnter

Add callback to onEnter entry in API docs

docs/API.md

@@ -255,9 +255,11 @@ code-splitting.
 ##### `children`
 Routes can be nested, `this.props.children` will contain the element created from the child route component. Please refer to the [Route Configuration](/docs/guides/basics/RouteConfiguration.md) since this is a very critical part of the router's design.
 
-##### `onEnter(nextState, replaceState)`
+##### `onEnter(nextState, replaceState, callback?)`
 Called when a route is about to be entered. It provides the next router state and a function to redirect to another path.
 
+If `callback` is listed as a 3rd argument, this hook will run asynchronously, and the transition will block until `callback` is called.
+
 ##### `onLeave()`
 Called when a route is about to be exited.