Refactor common code from google.oauth2.flow to google.oauth2.oauthlib (#106)

diff --git a/docs/reference/google.oauth2.rst b/docs/reference/google.oauth2.rst
index 5dc2406..4dd1ebd 100644
--- a/docs/reference/google.oauth2.rst
+++ b/docs/reference/google.oauth2.rst
@@ -14,5 +14,6 @@
    google.oauth2.credentials
    google.oauth2.flow
    google.oauth2.id_token
+   google.oauth2.oauthlib
    google.oauth2.service_account