codes: update docstring to indicate expected usage (#6701)

Author: dfawley

codes/codes.go

@@ -25,7 +25,13 @@ import (
 	"strconv"
 )
 
-// A Code is an unsigned 32-bit error code as defined in the gRPC spec.
+// A Code is a status code defined according to the [gRPC documentation].
+//
+// Only the codes defined as consts in this package are valid codes. Do not use
+// other code values.  Behavior of other codes is implementation-specific and
+// interoperability between implementations is not guaranteed.
+//
+// [gRPC documentation]: https://github.com/grpc/grpc/blob/master/doc/statuscodes.md
 type Code uint32
 
 const (