add comment about Xact limitations

Patch for "plugin: Add transactions for speculative operations"

pkg/util/xact/xact.go

@@ -8,6 +8,9 @@ type Xact[T any] struct {
 }
 
 // New returns a new transaction object (called Xact) operating on the given pointer
+//
+// NOTE: Any copying is shallow -- if T contains pointers, any changes to the values behind those
+// will NOT be delayed until (*Xact[T]).Commit().
 func New[T any](ptr *T) *Xact[T] {
 	return &Xact[T]{
 		tmp:  *ptr,