docs: fix document format for __next_private_export_map__ (#54952)

Fix the document format of `__next_private_export_map__`

follow up: #54695

packages/next/src/build/webpack/loaders/next-barrel-loader.ts

@@ -16,7 +16,10 @@
  * be `['a']`. Then, it request the "__barrel_transform__" SWC transform to load
  * `foo.js` and receive the following output:
  *
- *   export const __next_private_export_map__ = '[["./a","a","a"],["./b","b","b"],["./c","c","c"],...]'
+ *   export const __next_private_export_map__ = '[["a","./a","a"],["b","./b","b"],["c","./c","c"],...]'
+ *
+ *   format: '["<imported identifier>", "<import path>", "<exported name>"]'
+ *   e.g.: import { a as b } from './module-a' => '["b", "./module-a", "a"]'
  *
  * The export map, generated by SWC, is a JSON that represents the exports of
  * that module, their original file, and their original name (since you can do