chore(admin): clarify download points purchase mode copy

Rename residual 积分扣费/按次扣费 labels to 积分购买模式 and note cashier unlock; keep download_credit key for storage.

inc/classes/settings/class-aksrt-settings-credits-payment-groups.php

@@ -62,7 +62,7 @@ trait AKSRT_Settings_Credits_Payment_Groups {
     }
 
     /**
-     * 积分功能是否启用（开关 + cms_features.download_credit 联合判定）。
+     * 积分功能是否启用（仅积分系统开关）。
      */
     public static function credits_enabled(): bool {
         return AKSRT_Settings_Credits_Runtime::credits_enabled();