module: document parentURL in register options

doc/api/module.md

@@ -103,6 +103,10 @@ changes:
   URL, such as `import.meta.url`, you can pass that URL here. **Default:**
   `'data:'`
 * `options` {Object}
+  * `parentURL` {string|URL} If you want to resolve `specifier` relative to a
+    base URL, such as `import.meta.url`, you can pass that URL here. This
+    property is ignored if the `parentURL` is supplied as the second argument.
+    **Default:** `'data:'`
   * `data` {any} Any arbitrary, cloneable JavaScript value to pass into the
     [`initialize`][] hook.
   * `transferList` {Object\[]} [transferrable objects][] to be passed into the