docs(backup): add usage text to describe argument (#1193)

internal/commands/backup.go

@@ -25,6 +25,9 @@ import (
 var BackupCommand = cli.Command{
 	Name:  "backup",
 	Usage: "Creates album and index backups",
+	UsageText: `The first argument (if provided) is the path where the backup will be created.
+If not specified, a backup is created at PHOTOPRISM_BACKUP_PATH in a file named
+with today's date.`,
 	Flags:  backupFlags,
 	Action: backupAction,
 }