test&doc: add example and update doc for xerror package

xerror/xerror.go

@@ -6,6 +6,7 @@ package xerror
 
 // Unwrap if err is nil then it returns a valid value
 // If err is not nil, Unwrap panics with err.
+// Play: https://go.dev/play/p/w84d7Mb3Afk
 func Unwrap[T any](val T, err error) T {
 	if err != nil {
 		panic(err)