Improve help message wording

If're an admin and mess up the `users` list, you won't see "owner"
commands.

If you're just a regular room user (not an admin, not an owner), you'll
only see the `help` command in the `help` message.

Both of these situations may make you wonder:

- is that all there is?
- earlier I saw more commands, so what's going on?

Adding "and accessible to you" hopefully clears things up, or at least
it tries to make the help message more correct.

bot/command.go

@@ -191,7 +191,7 @@ func (b *Bot) sendHelp(ctx context.Context) {
 	}
 
 	var msg strings.Builder
-	msg.WriteString("The following commands are supported:\n\n")
+	msg.WriteString("The following commands are supported and accessible to you:\n\n")
 	for _, cmd := range b.commands {
 		if !cmd.allowed(evt.Sender, evt.RoomID) {
 			continue