base: clarify use of resource_names in DiscoveryRequest. (#130)

api/base.proto

@@ -116,7 +116,10 @@ message DiscoveryRequest {
   Node node = 2;
   // List of resources to subscribe to, e.g. list of cluster names or a route
   // configuration name. If this is empty, all resources for the API are
-  // returned.
+  // returned. LDS/CDS expect empty resource_names, since this is global
+  // discovery for the Envoy instance. The LDS and CDS responses will then imply
+  // a number of resources that need to be fetched via EDS/RDS, which will be
+  // explicitly enumerated in resource_names.
   repeated string resource_names = 3;
   // Type of the resource that is being requested, e.g.
   // "type.googleapis.com/envoy.api.v2.ClusterLoadAssignment". This is implicit