Update docs to indicate service class must be globally unique

ni/measurementlink/discovery/v1/discovery_service.proto

@@ -75,7 +75,7 @@ message ServiceDescriptor {
   // Registration can use the gRPC metadata to provide these names.
   repeated string provided_interfaces = 3;
 
-  // Required. The "class" of a service. The value of this field should be unique for a given interface in provided_interfaces.
+  // Required. The "class" of a service. The value of this field should be unique for all services.
   // In effect, the .proto service declaration defines the interface, and this field defines a class or concrete type of the interface.
   string service_class = 4;