doc: fix ESM/CJS wrapper example

PR-URL: #34853
Refs: #34714
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>

doc/api/esm.md

@@ -637,6 +637,12 @@ CommonJS entry point for `require`.
 }
 ```
 
+The above example uses explicit extensions `.mjs` and `.cjs`.
+If your files use the `.js` extension, `"type": "module"` will cause such files
+to be treated as ES modules, just as `"type": "commonjs"` would cause them
+to be treated as CommonJS.
+See [Enabling](#esm_enabling).
+
 ```js
 // ./node_modules/pkg/index.cjs
 exports.name = 'value';