fix: fix css.loaderOptions.css.modules default values (#4363)

fixes #4357

docs/guide/css.md

@@ -108,8 +108,13 @@ module.exports = {
   css: {
     loaderOptions: {
       css: {
-        localIdentName: '[name]-[hash]',
-        camelCase: 'only'
+        // Note: the following config format is different between Vue CLI v4 and v3
+        // For Vue CLI v3 users, please refer to css-loader v1 documentations
+        // https://github.com/webpack-contrib/css-loader/tree/v1.0.1
+        modules: {
+          localIdentName: '[name]-[hash]',
+          localsConvention: 'camelCaseOnly'
+        }
       }
     }
   }