Clarify use cases where gRPC's ALTS libraries are supported. (#1464)

See b/416011089.

Author: matthewstevenson88

layouts/shortcodes/docs/auth_alts.md

@@ -18,9 +18,9 @@ ALTS in gRPC has the following features:
 gRPC users can configure their applications to use ALTS as a transport security
 protocol with few lines of code.
 
-Note that ALTS is fully functional if the application runs on 
+Note that ALTS is fully supported for connections between GCP and Google if the application runs on 
 [Compute Engine](https://cloud.google.com/compute) or
-[Google Kubernetes Engine (GKE)](https://cloud.google.com/kubernetes-engine).
+[Google Kubernetes Engine (GKE)](https://cloud.google.com/kubernetes-engine). ALTS does not support workload-to-workload authentication between GCP workloads.
 
 {{ with .Page.Params.code.client_credentials -}}