Fix description of subcommand more consistent

[itchyny]

What type of PR is this?

(REQUIRED)

• bug
• cleanup
• documentation
• feature

What this PR does / why we need it:

Consider a cli with subcommands. I want to specify descriptions for both cmd task -h and cmd task list -h.

package main

import (
	"fmt"
	"log"
	"os"

	"github.com/urfave/cli/v2"
)

func main() {
	app := &cli.App{
		Commands: []*cli.Command{
			&cli.Command{
				Name:    "task",
				Aliases: []string{"a"},
				Usage:   "task commands",
				Description: `This is a description of task.
   For example, bla bla bla...`,
				Action: func(c *cli.Context) error {
					fmt.Println("tasks")
					return nil
				},
				Subcommands: []*cli.Command{
					&cli.Command{
						Name:  "list",
						Usage: "list tasks",
						Description: `This is a description of listing tasks.
   For example, bla bla bla...`,
						Action: func(c *cli.Context) error {
							fmt.Println("list tasks")
							return nil
						},
					},
				},
			},
		},
	}

	err := app.Run(os.Args)
	if err != nil {
		log.Fatal(err)
	}
}

Current behavior

When I run the command, I got the following results. Note that the description of go run . task -h appears at unexpected position.

 % go run . task -h
NAME:
   mycmd task - This is a description of task.
   For example, bla bla bla...

USAGE:
   mycmd task command [command options] [arguments...]

COMMANDS:
   list     list tasks
   help, h  Shows a list of commands or help for one command

OPTIONS:
   --help, -h  show help (default: false)

 % go run . task list -h # is fine!
NAME:
   mycmd task list - list tasks

USAGE:
   mycmd task list [command options] [arguments...]

DESCRIPTION:
   This is a description of listing tasks.
   For example, bla bla bla...

OPTIONS:
   --help, -h  show help (default: false)

Expected bahavior

I think this is more consistent. Shows both [command] - {{Usage}} and DESCRIPTION: {{Description}}.

 % go run . task -h
NAME:
   mycmd task - task commands

USAGE:
   mycmd task command [command options] [arguments...]

DESCRIPTION:
   This is a description of task.
   For example, bla bla bla...

COMMANDS:
   list     list tasks
   help, h  Shows a list of commands or help for one command

OPTIONS:
   --help, -h  show help (default: false)

 % go run . task list -h
NAME:
   mycmd task list - list tasks

USAGE:
   mycmd task list [command options] [arguments...]

DESCRIPTION:
   This is a description of listing tasks.
   For example, bla bla bla...

OPTIONS:
   --help, -h  show help (default: false)

Which issue(s) this PR fixes:

This pull request fixes the above problem.

Special notes for your reviewer:

Testing

(fill-in or delete this section)

Release Notes

(REQUIRED)

[codecov]

Codecov Report

Merging #1054 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1054   +/-   ##
=======================================
  Coverage   72.83%   72.83%           
=======================================
  Files          33       33           
  Lines        2474     2474           
=======================================
  Hits         1802     1802           
  Misses        565      565           
  Partials      107      107

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[saschagrunert]

LGTM