etcdctl: fill out ArgsUsage fields for help

USAGE in help now names positional arguments (e.g., "member remove <memberID>"
instead of "member remove [arguments...]")

Fixes #4021

etcdctl/command/set_dir_command.go

@@ -22,8 +22,9 @@ import (
 // NewSetDirCommand returns the CLI command for "setDir".
 func NewSetDirCommand() cli.Command {
 	return cli.Command{
-		Name:  "setdir",
-		Usage: "create a new or existing directory",
+		Name:      "setdir",
+		Usage:     "create a new or existing directory",
+		ArgsUsage: "<key>",
 		Flags: []cli.Flag{
 			cli.IntFlag{Name: "ttl", Value: 0, Usage: "key time-to-live"},
 		},