build: fix warnings in docgen (#711)

Resolved all the outstanding warnings from sphinx.

Going forward, warnings will be treated as errors since the `-W` flag is being used with `sphinx-build` 
diff --git a/docs/reference/google.auth.rst b/docs/reference/google.auth.rst
index e21eaf9..eb8328a 100644
--- a/docs/reference/google.auth.rst
+++ b/docs/reference/google.auth.rst
@@ -33,4 +33,4 @@
    google.auth.identity_pool
    google.auth.impersonated_credentials
    google.auth.jwt
-   google.auth.jwt_async
+   google.auth._jwt_async