feat: add reauth support to async user credentials (#738)

diff --git a/google/oauth2/reauth.py b/google/oauth2/reauth.py
index d539d7c..d914fe9 100644
--- a/google/oauth2/reauth.py
+++ b/google/oauth2/reauth.py
@@ -296,9 +296,9 @@
         rapt_token (Optional(str)): The rapt token for reauth.
 
     Returns:
-        Tuple[str, Optional[str], Optional[datetime], Mapping[str, str]]: The
-            access token, new refresh token, expiration, and additional data
-            returned by the token endpoint.
+        Tuple[str, Optional[str], Optional[datetime], Mapping[str, str], str]: The
+            access token, new refresh token, expiration, the additional data
+            returned by the token endpoint, and the rapt token.
 
     Raises:
         google.auth.exceptions.RefreshError: If the token endpoint returned