Fix incorrect link in README to auth state changes (#687)

* Fix incorrect link in README to auth state changes

* Add clarification to README

* Fix typo

Author: SUPERCILEX

auth/README.md

@@ -263,9 +263,9 @@ protected void onActivityResult(int requestCode, int resultCode, Intent data) {
 ```
 
 Alternatively, you can register a listener for authentication state changes;
-see the
-[Firebase Auth documentation](https://firebase.google.com/docs/auth/android/manage-users#get_the_currently_signed-in_user)
-for more information.
+see the Firebase Auth documentation to
+[get the currently signed-in user](https://firebase.google.com/docs/auth/android/manage-users#get_the_currently_signed-in_user)
+and [register an AuthStateListener](https://firebase.google.com/docs/reference/android/com/google/firebase/auth/FirebaseAuth.html#addAuthStateListener(com.google.firebase.auth.FirebaseAuth.AuthStateListener)).
 
 ##### ID Tokens
 To retrieve the ID token that the IDP returned, you can extract an `IdpResponse` from the result