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.downscoped.rst b/docs/reference/google.auth.downscoped.rst
new file mode 100644
index 0000000..79668f9
--- /dev/null
+++ b/docs/reference/google.auth.downscoped.rst
@@ -0,0 +1,7 @@
+google.auth.downscoped module
+=============================
+
+.. automodule:: google.auth.downscoped
+   :members:
+   :inherited-members:
+   :show-inheritance: