Clarify command descriptions when no arguments are taken (#496)

This changes the command help string from eg
```
NAME:
   tea label create - Create a label

USAGE:
   tea label [command options] [arguments...]
```
to
```
NAME:
   tea label create - Create a label

USAGE:
   tea label [command options]
```

Hopefully improving usability.

---

edit: this also changes `tea release create` to take the `--tag` flag value optionally via the first argument, as this seems to be a clear UX improvement.

fixes #483

Co-authored-by: Norwin <git@nroo.de>
Co-authored-by: 6543 <6543@obermui.de>
Reviewed-on: https://gitea.com/gitea/tea/pulls/496
Reviewed-by: techknowlogick <techknowlogick@gitea.io>
Reviewed-by: 6543 <6543@obermui.de>
Reviewed-by: delvh <delvh@noreply.gitea.io>
Co-authored-by: Norwin <noerw@noreply.gitea.io>
Co-committed-by: Norwin <noerw@noreply.gitea.io>

cmd/notifications/list.go

@@ -24,6 +24,7 @@ var CmdNotificationsList = cli.Command{
 	Aliases:     []string{"list"},
 	Usage:       "List notifications",
 	Description: `List notifications`,
+	ArgsUsage:   " ", // command does not accept arguments
 	Action:      RunNotificationsList,
 	Flags:       append([]cli.Flag{notifTypeFlag}, flags.NotificationFlags...),
 }