etcdctl: add ttl unit to flag description

Add the ttl unit (seconds) to --ttl description for etcdctl's mk, mkdir, set,
setdir, update, and updatedir commands.

etcdctl/ctlv2/command/set_dir_command.go

@@ -26,7 +26,7 @@ func NewSetDirCommand() cli.Command {
 		Usage:     "create a new directory or update an existing directory TTL",
 		ArgsUsage: "<key>",
 		Flags: []cli.Flag{
-			cli.IntFlag{Name: "ttl", Value: 0, Usage: "key time-to-live"},
+			cli.IntFlag{Name: "ttl", Value: 0, Usage: "key time-to-live in seconds"},
 		},
 		Action: func(c *cli.Context) error {
 			mkdirCommandFunc(c, mustNewKeyAPI(c), client.PrevIgnore)