Adding links to the Firebase reference guide

Author: jamesdaniels

docs/version-4-upgrade.md

@@ -25,7 +25,7 @@ constructor(db: AngularFireDatabase) {
 
 ### Simplified Authentication API
 
-In 4.0 we've further cut the complexity of the package by auth module down to an obverser for state changes to simplify the package.
+In 4.0 we've further cut the complexity of the package by auth module down to a [`firebase.User`](https://firebase.google.com/docs/reference/js/firebase.User) observer to simplify the package.
 
 ```
 import { AngularFireAuth } from 'angularfire2/auth';
@@ -53,6 +53,8 @@ logout() {
 }
 ```
 
+(See what methods are available to `firebase.auth.Auth` in the Firebase reference guide)[https://firebase.google.com/docs/reference/js/firebase.auth.Auth].
+
 ### FirebaseListFactory and FirebaseObjectFactory API Changes
 
 If you directly use `FirebaseListFactory` or `FirebaseObjectFactory` you will no longer be able to pass in a string, it will instead expect a Firebase database reference.