Docs: note ValueError in verify_oauth2_token (#928)

These `ValueError`s can be raised by `google.auth.jwt.decode` e.g.:

* https://github.com/googleapis/google-auth-library-python/blob/8f1e9cfd56dbaae0dff64499e1d0cf55abc5b97e/google/oauth2/id_token.py#L162
* https://github.com/googleapis/google-auth-library-python/blob/8f1e9cfd56dbaae0dff64499e1d0cf55abc5b97e/google/oauth2/id_token.py#L135
* https://github.com/googleapis/google-auth-library-python/blob/8f1e9cfd56dbaae0dff64499e1d0cf55abc5b97e/google/auth/jwt.py#L255

Handling of this is noted in the user docs [1]

[1] https://developers.google.com/identity/sign-in/web/backend-auth

Co-authored-by: arithmetic1728 <58957152+arithmetic1728@users.noreply.github.com>

Author: matthewhughes934

google/oauth2/id_token.py

@@ -158,6 +158,7 @@ def verify_oauth2_token(id_token, request, audience=None, clock_skew_in_seconds=
 
     Raises:
         exceptions.GoogleAuthError: If the issuer is invalid.
+        ValueError: If token verification fails
     """
     idinfo = verify_token(
         id_token,