include explicit example

Author: guybedford

doc/api/modules.md

@@ -238,8 +238,9 @@ To create an ESM module that should provide an arbitrary value to CommonJS, the
 ```mjs
 // point.mjs
 export default class Point {
-  constructor(c, x) { this.x = x; this.y = y; }
+  constructor(x, y) { this.x = x; this.y = y; }
 }
+export function distance(a, b) { return (b.x - a.x) ** 2 + (b.y - a.y) ** 2; }
 export const cjsUnwrapDefault = true;
 ```
 
@@ -250,7 +251,9 @@ console.log(point);
 ```
 
 When using `cjsUnwrapDefault`, no named exports other than the `default` export
-will be accessible to CommonJS importers.
+will be accessible to CommonJS importers - `distance` is not available for CJS
+consumers in the above `require('./point.mjs')`. Instead it should be made a
+property of the class (e.g. as a static method).
 
 If the module being `require()`'d contains top-level `await`, or the module
 graph it `import`s contains top-level `await`,