chore(cmd): print shortHelp if longHelp is empty

Currenlty, `shortHelp` is used when the command is listed from its
parent command, and `longHelp` is used when the command is executed with
its help flag.

What I found a little annoying is if `longHelp` is not provided, which
is the case for the majority of commands, there's no text at all when
the command is invoked with the help flag.

This change ensures the `shortHelp` is printed if `longHelp` is empty.

gnovm/cmd/gno/doc.go

@@ -28,7 +28,7 @@ func newDocCmd(io *commands.IO) *commands.Command {
 		commands.Metadata{
 			Name:       "doc",
 			ShortUsage: "doc [flags] <pkgsym>",
-			ShortHelp:  "get documentation for the specified package or symbol (type, function, method, or variable/constant).",
+			ShortHelp:  "get documentation for the specified package or symbol (type, function, method, or variable/constant)",
 		},
 		c,
 		func(_ context.Context, args []string) error {

tm2/pkg/commands/command.go

@@ -252,6 +252,8 @@ func usage(c *Command) string {
 
 	if c.longHelp != "" {
 		fmt.Fprintf(&b, "%s\n\n", c.longHelp)
+	} else {
+		fmt.Fprintf(&b, "%s.\n\n", c.shortHelp)
 	}
 
 	if len(c.subcommands) > 0 {