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

xerror/xerror_example_test.go

@@ -0,0 +1,25 @@
+package xerror
+
+import (
+	"fmt"
+	"reflect"
+	"strconv"
+)
+
+func ExampleUnwrap() {
+	result1 := Unwrap(strconv.Atoi("42"))
+	fmt.Println(result1)
+
+	_, err := strconv.Atoi("4o2")
+	defer func() {
+		v := recover()
+		result2 := reflect.DeepEqual(err.Error(), v.(*strconv.NumError).Error())
+		fmt.Println(result2)
+	}()
+
+	Unwrap(strconv.Atoi("4o2"))
+
+	// Output:
+	// 42
+	// true
+}