[modules] Add comment explaining synchronous error handling

Bug: v8:5785
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Change-Id: I1792c57c37bac996079d5ecce5b61a13b327fb54
Reviewed-on: https://chromium-review.googlesource.com/596467Reviewed-by: Adam Klein <adamk@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Sathya Gunasekaran <gsathya@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47056}

include/v8.h

@@ -6198,7 +6198,9 @@ typedef void (*DeprecatedCallCompletedCallback)();
  * The Promise returned from this function is forwarded to userland
  * JavaScript. The embedder must resolve this promise with the module
  * namespace object. In case of an exception, the embedder must reject
- * this promise with the exception.
+ * this promise with the exception. If the promise creation itself
+ * fails (e.g. due to stack overflow), the embedder must propagate
+ * that exception by returning an empty MaybeLocal.
  */
 typedef MaybeLocal<Promise> (*HostImportModuleDynamicallyCallback)(
     Local<Context> context, Local<String> referrer, Local<String> specifier);