Small clarification on dict and filepath GCP tokens (#603)

Fix gh-583

Author: astrojuanlu

docs/source/index.rst

@@ -99,7 +99,7 @@ Several modes of authentication are supported:
       or container engine) and fetch the credentials automatically from the
       metadata service.
 
-    - you may supply a token generated by the
+    - if ``token=dict(...)`` or ``token=<filepath>``, you may supply a token generated by the
       gcloud_ utility; this is either a python dictionary, or the name of a file
       containing the JSON returned by logging in with the gcloud CLI tool (e.g.,
       ``~/.config/gcloud/application_default_credentials.json`` or