docs: update user guide/references for downscoped creds (#827)

Updates user guide to document using downscoping with
Credential Access Boundaries.
Regenerates references for all classes and utilities related
to this feature.

Co-authored-by: arithmetic1728 <58957152+arithmetic1728@users.noreply.github.com>
diff --git a/docs/reference/google.auth.rst b/docs/reference/google.auth.rst
index eb8328a..06cc267 100644
--- a/docs/reference/google.auth.rst
+++ b/docs/reference/google.auth.rst
@@ -26,6 +26,7 @@
    google.auth.aws
    google.auth.credentials
    google.auth._credentials_async
+   google.auth.downscoped
    google.auth.environment_vars
    google.auth.exceptions
    google.auth.external_account