docs(faq): add the most common mistake of config

docs/source/en/basics/config.md

@@ -73,6 +73,18 @@ root       | The application root directory, if the environment is local or unit
 
 `appInfo.root` is an elegant adaption. for example, we tend to use ``/home/admin/logs`` as the catalog of log in the server environment, while we don't want to pollute the user catalog in local development. This adaptation is very good at solving this problem.
 
+Choose the appropriate style according to the specific situation, but please make sure you don't make mistake like the code below:
+
+```js
+// config/config.default.js
+exports.someKeys = 'abc';
+module.exports = appInfo => {
+  const config = {};
+  config.keys = '123456';
+  return config;
+};
+```
+
 ### Sequence of loading configurations
 
 Applications, plugin components and framework are able to define those configs. Even though the structure of catalog is identical but there is priority (application > framework > plugin). Besides, the running environment has the higher priority.

docs/source/en/faq.md

@@ -21,6 +21,18 @@ Framework [Config](./basics/config.md) settings is powerfull, support different
 
 When you got some trouble, and want to find out what is the final config using at runtime, you can checkout `${root}/run/application_config.json`(workers' configurations) and `${root}/run/agent_config.json`(agent's configurations).(`root` is application's root directory, in `local` and `unittest` environments, it will be project base directory, in other environments will be HOME directory)
 
+Please make sure you don't make mistake like the code below:
+
+```js
+// config/config.default.js
+exports.someKeys = 'abc';
+module.exports = appInfo => {
+  const config = {};
+  config.keys = '123456';
+  return config;
+};
+```
+
 ## Where are my log files in prod environment?
 
 By default, logs will print at `${baseDir}/logs`(baseDir is project's base directory) in local environment.But in non-development environments(neither local nor unittest), the logs will print at `$HOME/logs`(such as `/home/admin/logs`). So the logs won't mix in during development and locate in the same place when run in production environment.

docs/source/zh-cn/basics/config.md

@@ -76,6 +76,19 @@ root | 应用根目录，只有在 local 和 unittest 环境下为 baseDir，其
 
 `appInfo.root` 是一个优雅的适配，比如在服务器环境我们会使用 `/home/admin/logs` 作为日志目录，而本地开发时又不想污染用户目录，这样的适配就很好解决这个问题。
 
+
+请根据具体场合选择合适的写法，但请确保没有写出以下代码：
+
+```js
+// config/config.default.js
+exports.someKeys = 'abc';
+module.exports = appInfo => {
+  const config = {};
+  config.keys = '123456';
+  return config;
+};
+```
+
 ### 配置加载顺序
 
 应用、插件、框架都可以定义这些配置，而且目录结构都是一致的，但存在优先级（应用 > 框架 > 插件），相对于此运行环境的优先级会更高。

docs/source/zh-cn/faq.md

@@ -23,6 +23,18 @@ title: 常见问题
 
 也可参见[配置文件](https://eggjs.org/zh-cn/basics/config.html#配置结果)。
 
+PS：请确保没有写出以下代码：
+
+```js
+// config/config.default.js
+exports.someKeys = 'abc';
+module.exports = appInfo => {
+  const config = {};
+  config.keys = '123456';
+  return config;
+};
+```
+
 ## 线上的日志打印去哪里了？
 
 默认配置下，本地开发环境的日志都会打印在应用根目录的 `logs` 文件夹下(`${baseDir}/logs`) ，但是在非开发期的环境（非 local 和 unittest 环境），所有的日志都会打印到 `$HOME/logs` 文件夹下（例如 `/home/admin/logs`）。这样可以让本地开发时应用日志互不影响，服务器运行时又有统一的日志输出目录。