Generate Help on a specific verb

使用所示的 Verb1 和 Verb2 类复现该问题，然后跟踪结果从 ParseArguments 经过 DisplayHelp、HelpText.AutoBuild 和 DefaultParsingErrorsHandler 的处理过程。检查如何解释 "help verb1" 的参数，以及 verbsIndex 如何影响输出。完成标准是：请求某个 verb 的帮助时，只显示该 verb 的帮助和选项，同时保留现有的所有 verb 帮助行为。

Describe the bug
When I call my program with the following arguments : "help verb1", I expect that the help for this specific verb was displayed. But instead, the help for all verbs are displayed.

To Reproduce

    [Verb("Verb1", HelpText ="Help Verb1")]
    internal class Verb1
    {
        [Option(Required = true, HelpText = "Help MyOption1")]
        public string MyOption1 { get; set; }
    }

    [Verb("Verb2",HelpText = "Help Verb2")]
    internal class Verb2
    {
        [Option(Required = true, HelpText = "Help MyOption2")]
        public string MyOption2 { get; set; }
    }

    internal class Program
    {
        static void Main(string[] args)
        {
            var parser = new Parser(config => config.HelpWriter = null);
            var parserResult = parser.ParseArguments<
                Verb1,
                Verb2
            >
            (args);

            parserResult.MapResult(
                 (Verb1 options) => { return 0; },
                 (Verb2 options) => { return 0; },
                 errors => {
                     DisplayHelp(parserResult, errors);
                     return 0;
                 });

            Console.ReadKey();
        }

        static void DisplayHelp<T>(ParserResult<T> result, IEnumerable<Error> errs)
        {
            var helpText = HelpText.AutoBuild(result, h =>
            {
                h.AdditionalNewLineAfterOption = false;
                h.Heading = "Myapp 2.0.0-beta"; //change header
                h.Copyright = "Copyright (c) 2019 Global.com"; //change copyright text
                return HelpText.DefaultParsingErrorsHandler(result, h);
            }, e => e, verbsIndex:true);
            Console.WriteLine(helpText);
        }
    }

When I excecute this with the command line "MyApp.exe help verb1"
The actual result is :

Expected behavior
This display :
Verb2 Help Verb1
MyOption2 Help MyOption2