docs: add some comments on dest (#1357)

Author: shigma

packages/docs/docs/config/README.md

@@ -70,7 +70,7 @@ Specify the port to use for the dev server.
 - Type: `string`
 - Default: `.vuepress/dist`
 
-Specify the output directory for `vuepress build`.
+Specify the output directory for `vuepress build`. If a relative path is specified, it will be resolved based on `process.cwd()`.
 
 ### ga
 

packages/docs/docs/zh/config/README.md

@@ -68,7 +68,7 @@ module.exports = {
 - 类型: `string`
 - 默认值: `.vuepress/dist`
 
-指定 `vuepress build` 的输出目录。
+指定 `vuepress build` 的输出目录。如果传入的是相对路径，则会基于 `process.cwd()` 进行解析。
 
 ### ga