Add super helpful explanation of previously confusing error-on-read

packages/scenario/test/serialization.test.ts

@@ -194,6 +194,23 @@ describe('ExternalSecondaryInstanceParseTest.java', () => {
  /**
  * **PORTING NOTES**
  *
+ * The original notes below were answered with
+ * {@link https:/getodk/web-forms/pull/110#discussion_r1614139373 | this excellent explanation}:
+ *
+ * > This is the result of a bunch of implementation details/decisions in JR
+ * > and Collect. As you've noted in some earlier tests, lists of select
+ * > options are not part of the DAG/recomputation model. Choice lists are
+ * > only computed when they need to be displayed. In some cases, this can
+ * > result in a significant perf improvement.
+ * >
+ * > So the choicesOf call computes the choices. And that's fine even if the
+ * > references for label and name don't exist because that just returns the
+ * > entirety of every choice item. It's only when a selection is made that
+ * > there's an attempt to use the specified references and that causes a
+ * > crash.
+ *
+ * - - -
+ *
  * Expanding on the mental model we're trying to form in the skipped test
  * directly above...
  *