docs: add warnings on css sideEffects (#4836)

These warnings are originally stated in the vue-loader documentation,
but the CLI documentation perhaps have more visibility.

Fixes #4832

docs/guide/build-targets.md

@@ -50,6 +50,10 @@ A lib build outputs:
 
 - `dist/myLib.css`: Extracted CSS file (can be forced into inlined by setting `css: { extract: false }` in `vue.config.js`)
 
+::: warning
+If you are developing a library or in a monorepo, please be aware that CSS imports **are side effects**. Make sure to **remove** `"sideEffects": false` in the `package.json`, otherwise CSS chunks will be dropped by webpack in production builds.
+:::
+
 ### Vue vs. JS/TS Entry Files
 
 When using a `.vue` file as entry, your library will directly expose the Vue component itself, because the component is always the default export.
@@ -148,7 +152,7 @@ Now on the page, the user only needs to include Vue and the entry file:
 
 When building a [Webcomponent](#web-component) or [Library](#library), the entry point is not `main.js`, but an `entry-wc.js` file, generated here: [https://github.com/vuejs/vue-cli/blob/dev/packages/%40vue/cli-service/lib/commands/build/resolveWcEntry.js](https://github.com/vuejs/vue-cli/blob/dev/packages/%40vue/cli-service/lib/commands/build/resolveWcEntry.js)
 
-So to use vuex in web component target, you need to initialize the store in `App.vue`: 
+So to use vuex in web component target, you need to initialize the store in `App.vue`:
 
 ``` js
 import store from './store'

docs/zh/guide/build-targets.md

@@ -54,6 +54,10 @@ dist/myLib.css           0.33 kb                  0.23 kb
 
 - `dist/myLib.css`：提取出来的 CSS 文件 (可以通过在 `vue.config.js` 中设置 `css: { extract: false }` 强制内联)
 
+::: warning 警告
+如果你在开发一个库或多项目仓库 (monorepo)，请注意导入 CSS **是具有副作用的**。请确保在 `package.json` 中**移除** `"sideEffects": false`，否则 CSS 代码块会在生产环境构建时被 webpack 丢掉。
+:::
+
 ### Vue vs. JS/TS 入口文件
 
 当使用一个 `.vue` 文件作为入口时，你的库会直接暴露这个 Vue 组件本身，因为组件始终是默认导出的内容。