Update docs. Fill in some missed tests.

docs/API.md

@@ -350,7 +350,7 @@ Called on routes when the location changes, but the route itself neither enters
 
 If `callback` is listed as a 4th argument, this hook will run asynchronously, and the transition will block until `callback` is called.
 
-##### `onLeave()`
+##### `onLeave(prevState)`
 Called when a route is about to be exited.
 
 

docs/Glossary.md

@@ -66,10 +66,10 @@ A *hash* is a string that represents the hash portion of the URL. It is synonymo
 ## LeaveHook
 
 ```js
-type LeaveHook = () => any;
+type LeaveHook = (prevState: RouterState) => any;
 ```
 
-A *leave hook* is a user-defined function that is called when a route is about to be unmounted.
+A *leave hook* is a user-defined function that is called when a route is about to be unmounted. It receives the previous [router state](#routerstate) as its first argument. 
 
 ## Location
 

modules/__tests__/transitionHooks-test.js

@@ -104,8 +104,9 @@ describe('When a router enters a branch', function () {
         expect(nextState.routes).toContain(InboxRoute)
         expect(replace).toBeA('function')
       },
-      onLeave() {
+      onLeave(prevState) {
         expect(this).toBe(InboxRoute)
+        expect(prevState.routes).toContain(InboxRoute)
       }
     }
 
@@ -173,8 +174,9 @@ describe('When a router enters a branch', function () {
         expect(nextState.routes).toContain(DashboardRoute)
         expect(replace).toBeA('function')
       },
-      onLeave() {
+      onLeave(prevState) {
         expect(this).toBe(DashboardRoute)
+        expect(prevState.routes).toContain(DashboardRoute)
       },
       childRoutes: [ NewsFeedRoute, InboxRoute, RedirectToInboxRoute, MessageRoute, UserRoute ]
     }