Add GCS configuration info to Authoring Guide (#1737)

* Add GCS configuration info to Authoring Guide

Add information about the CLOUD_STORAGE_BUCKET environment variable used by tests, as well as the availability of cloud-samples-data which contains files required for a number of the test suites

* Remove cloud-samples-data from Authoring Guide

Remove info about cloud-samples-data from Authoring Guide because the tests that use this use it explicitly, not from $CLOUD_STORAGE_BUCKET

AUTHORING_GUIDE.md

@@ -478,11 +478,14 @@ need to set environment variables for the tests to be able to use your project
 and its resources. See `testing/test-env.tmpl.sh` for a list of all environment
 variables used by all tests. Not every test needs all of these variables.
 
+#### Google Cloud Storage resources
 
+Certain samples require integration with Google Cloud Storage (GCS),
+most commonly for APIs that read files from GCS. To run the tests for
+these samples, configure your GCS bucket name via the `CLOUD_STORAGE_BUCKET`
+environment variable.
 
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+The resources required by tests can usually be found in the `./resources`
+folder inside the sample directory. You can upload these resources to your
+own bucket to run the tests, e.g. using `gsutil`:  
+`gsutil cp ./resources/* gs://$CLOUD_STORAGE_BUCKET/`