docs: improve parallel option docs [ci skip]

yyx990803 · yyx990803 · commit eb97e48aea52 · 2018-08-14T10:21:42.000-04:00
diff --git a/docs/config/README.md b/docs/config/README.md
@@ -344,7 +344,7 @@ module.exports = {
 - Type: `boolean`
 - Default: `require('os').cpus().length > 1`
 
-  Whether to use `thread-loader` for Babel or TypeScript transpilation.
+  Whether to use `thread-loader` for Babel or TypeScript transpilation. This is enabled for production builds when the system has more than 1 CPU cores.
 
 ### pwa
 
diff --git a/docs/zh/config/README.md b/docs/zh/config/README.md
@@ -342,7 +342,7 @@ module.exports = {
 - Type: `boolean`
 - Default: `require('os').cpus().length > 1`
 
-  是否为 Babel 或 TypeScript 使用 `thread-loader`。
+  是否为 Babel 或 TypeScript 使用 `thread-loader`。该选项在系统的 CPU 有多于一个内核时自动启用，仅作用于生产构建。
 
 ### pwa
 
