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/rmdir_command.go

@@ -24,8 +24,9 @@ import (
 // NewRemoveCommand returns the CLI command for "rmdir".
 func NewRemoveDirCommand() cli.Command {
 	return cli.Command{
-		Name:  "rmdir",
-		Usage: "removes the key if it is an empty directory or a key-value pair",
+		Name:      "rmdir",
+		Usage:     "removes the key if it is an empty directory or a key-value pair",
+		ArgsUsage: "<key>",
 		Action: func(c *cli.Context) {
 			rmdirCommandFunc(c, mustNewKeyAPI(c))
 		},