cmd/tailscale/cli: document how to see subcommand usage

From user feedback.

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/tailscale/cli/cli.go

@@ -50,9 +50,13 @@ func Run(args []string) error {
 
 	rootCmd := &ffcli.Command{
 		Name:       "tailscale",
-		ShortUsage: "tailscale subcommand [flags]",
+		ShortUsage: "tailscale [flags] <subcommand> [command flags]",
 		ShortHelp:  "The easiest, most secure way to use WireGuard.",
 		LongHelp: strings.TrimSpace(`
+For help on subcommands, add -help after: "tailscale status -help".
+
+All flags can use single or double hyphen prefixes (-help or --help).
+
 This CLI is still under active development. Commands and flags will
 change in the future.
 `),

cmd/tailscale/cli/status.go

@@ -37,7 +37,7 @@ var statusCmd = &ffcli.Command{
 		fs.BoolVar(&statusArgs.active, "active", false, "filter output to only peers with active sessions (not applicable to web mode)")
 		fs.BoolVar(&statusArgs.self, "self", true, "show status of local machine")
 		fs.BoolVar(&statusArgs.peers, "peers", true, "show status of peers")
-		fs.StringVar(&statusArgs.listen, "listen", "127.0.0.1:8384", "listen address; use port 0 for automatic")
+		fs.StringVar(&statusArgs.listen, "listen", "127.0.0.1:8384", "listen address for web mode; use port 0 for automatic")
 		fs.BoolVar(&statusArgs.browser, "browser", true, "Open a browser in web mode")
 		return fs
 	})(),