docs: explain stylesheet ownership, responsive rules and display boundaries

app/assets/styles/MfCloudResourceUpgrade.bSwLWjPg.css

@@ -1,3 +1,5 @@
+/* 云资源升级：可选配置、资源选项与汇总金额。 */
+/* 导入方：app/components/instance/InstanceBilling.vue、app/components/mfcloud/MfCloudResourceUpgrade.vue。 */
 .resource-upgrade[data-v-c9212028] {
   gap: 24px;
 }
@@ -49,6 +51,7 @@
   margin: 0;
   font-size: 0.875rem;
 }
+/* 视口条件 (width<=640px) 下的响应式覆盖；保留相对基础规则的层叠顺序。 */
 @media (width<=640px) {
   .upgrade-total[data-v-c9212028] {
     flex-direction: column;