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.credentials_async.rst b/docs/reference/google.auth._credentials_async.rst
similarity index 100%
rename from docs/reference/google.auth.credentials_async.rst
rename to docs/reference/google.auth._credentials_async.rst
diff --git a/docs/reference/google.auth.jwt_async.rst b/docs/reference/google.auth._jwt_async.rst
similarity index 75%
rename from docs/reference/google.auth.jwt_async.rst
rename to docs/reference/google.auth._jwt_async.rst
index 4e56a6e..d27984b 100644
--- a/docs/reference/google.auth.jwt_async.rst
+++ b/docs/reference/google.auth._jwt_async.rst
@@ -1,7 +1,7 @@
 google.auth.jwt\_async module
 =============================
 
-.. automodule:: google.auth.jwt_async
+.. automodule:: google.auth._jwt_async
    :members:
    :inherited-members:
    :show-inheritance:
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
diff --git a/docs/reference/google.auth.transport.aiohttp_requests.rst b/docs/reference/google.auth.transport._aiohttp_requests.rst
similarity index 100%
rename from docs/reference/google.auth.transport.aiohttp_requests.rst
rename to docs/reference/google.auth.transport._aiohttp_requests.rst
diff --git a/docs/reference/google.oauth2.credentials_async.rst b/docs/reference/google.oauth2._credentials_async.rst
similarity index 100%
rename from docs/reference/google.oauth2.credentials_async.rst
rename to docs/reference/google.oauth2._credentials_async.rst
diff --git a/docs/reference/google.oauth2.service_account_async.rst b/docs/reference/google.oauth2._service_account_async.rst
similarity index 100%
rename from docs/reference/google.oauth2.service_account_async.rst
rename to docs/reference/google.oauth2._service_account_async.rst