docs(admin): clarify subscription quota uses margin multiplier in billing UI

Co-authored-by: Cursor <cursoragent@cursor.com>

src/admin/pages/BillingPage.tsx

@@ -326,7 +326,8 @@ function FormulaConfigForm({
       </label>
 
       <p className="muted">
-        预览：成本模式扣费 ≈ 上游 USD × {draft.usdCnyRate} × {draft.marginMultiplier}
+        预览：成本模式扣费 ≈ 上游 USD × {draft.usdCnyRate} × {draft.marginMultiplier}；
+        套餐额度扣减 ≈ 实际上游 Token × {draft.marginMultiplier}
       </p>
 
       <button type="submit" className="send-btn" disabled={saving || envLocked}>