如何使用godoc正确生成文档？

Here is my code and terminal window. What should I do to make it work well and return function description?

// Copyright DOCUMENTATION FOR ME. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package testDoc

// sayHello return "Hello world string"
func sayHello() string{
    return "Hello world!"
}

terminal return

Use the -u flag to show unexported symbols: go doc -u ./test