swarming: use subcommands.Section

This shows help like
```
Client tool to access a swarming server.

Usage:  swarming [command] [arguments]

Commands:

        task related commands

  cancel        cancel a task
  collect       Waits on a set of Swarming tasks
  reproduce     reproduces a task locally
  request-show  returns properties of a request
  spawn-tasks   Spawns a set of Swarming tasks
  tasks         lists tasks
  trigger       Triggers a Swarming task

        bot related commands

  bots          lists bots
  delete-bots   delete bots
  terminate     terminate a bot

        other commands

  help          prints help about a command
  whoami        prints an email address associated with currently cached token
  login         performs interactive login flow
  logout        removes cached credentials
  version       prints the executable version


Use "swarming help [command]" for more information about a command.
Use "swarming help -advanced" to display all commands.
```

Change-Id: I6f91384cc2772ae9a8ff8555016a627b4334d06e
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/3247471
Auto-Submit: Takuto Ikuta <tikuta@chromium.org>
Commit-Queue: Junji Watanabe <jwata@google.com>
Reviewed-by: Junji Watanabe <jwata@google.com>
diff --git a/client/cmd/swarming/main.go b/client/cmd/swarming/main.go
index 1d7e85f..fa9b52b 100644
--- a/client/cmd/swarming/main.go
+++ b/client/cmd/swarming/main.go
@@ -83,16 +83,19 @@
 		Title: "Client tool to access a swarming server.",
 		// Keep in alphabetical order of their name.
 		Commands: []*subcommands.Command{
-			lib.CmdBots(af),
+			subcommands.Section("task related commands\n"),
 			lib.CmdCancelTask(af),
 			lib.CmdCollect(af),
-			lib.CmdDeleteBots(af),
 			lib.CmdReproduce(af),
 			lib.CmdRequestShow(af),
 			lib.CmdSpawnTasks(af),
 			lib.CmdTasks(af),
-			lib.CmdTerminateBot(af),
 			lib.CmdTrigger(af),
+			subcommands.Section("bot related commands\n"),
+			lib.CmdBots(af),
+			lib.CmdDeleteBots(af),
+			lib.CmdTerminateBot(af),
+			subcommands.Section("other commands\n"),
 			subcommands.CmdHelp,
 			authcli.SubcommandInfo(authOpts, "whoami", false),
 			authcli.SubcommandLogin(authOpts, "login", false),