docs: add thread-loader warnings (#5777) [ci skip]

docs/config/README.md

@@ -380,6 +380,10 @@ In v3 this means the opposite of `css.requireModuleExtension`.
 
   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. Passing a number will define the amount of workers used.
 
+::: warning
+Do not use `parallel` in combination with non-serializable loader options, such as regexes, dates and functions. These options would not be passed correctly to the respective loaders which may lead to unexpected errors.
+:::
+
 ### pwa
 
 - Type: `Object`

docs/core-plugins/babel.md

@@ -26,6 +26,8 @@ module.exports = {
 
 [thread-loader](https://github.com/webpack-contrib/thread-loader) is enabled by default when the machine has more than 1 CPU cores. This can be turned off by setting `parallel: false` in `vue.config.js`.
 
+`parallel` should be set to `false` when using Babel in combination with non-serializable loader options, such as regexes, dates and functions. These options would not be passed correctly to `babel-loader` which may lead to unexpected errors.
+
 ## Installing in an Already Created Project
 
 ``` sh

docs/core-plugins/typescript.md

@@ -24,6 +24,8 @@ If opted to use [TSLint](https://palantir.github.io/tslint/) during project crea
 
 [thread-loader](https://github.com/webpack-contrib/thread-loader) is enabled by default when the machine has more than 1 CPU cores. This can be turned off by setting `parallel: false` in `vue.config.js`.
 
+`parallel` should be set to `false` when using Typescript in combination with non-serializable loader options, such as regexes, dates and functions. These options would not be passed correctly to `ts-loader` which may lead to unexpected errors.
+
 ## Installing in an Already Created Project
 
 ``` sh

packages/@vue/cli-plugin-babel/README.md

@@ -26,6 +26,8 @@ module.exports = {
 
 [thread-loader](https://github.com/webpack-contrib/thread-loader) is enabled by default when the machine has more than 1 CPU cores. This can be turned off by setting `parallel: false` in `vue.config.js`.
 
+`parallel` should be set to `false` when using Babel in combination with non-serializable loader options, such as regexes, dates and functions. These options would not be passed correctly to `babel-loader` which may lead to unexpected errors.
+
 ## Installing in an Already Created Project
 
 ``` sh

packages/@vue/cli-plugin-typescript/README.md

@@ -24,6 +24,8 @@ If opted to use [TSLint](https://palantir.github.io/tslint/) during project crea
 
 [thread-loader](https://github.com/webpack-contrib/thread-loader) is enabled by default when the machine has more than 1 CPU cores. This can be turned off by setting `parallel: false` in `vue.config.js`.
 
+`parallel` should be set to `false` when using Typescript in combination with non-serializable loader options, such as regexes, dates and functions. These options would not be passed correctly to `ts-loader` which may lead to unexpected errors.
+
 ## Installing in an Already Created Project
 
 ``` sh