XFA: Add a comment regarding the shut down order.

R=tsepez@chromium.org

Review URL: https://codereview.chromium.org/1234323003 .
diff --git a/samples/pdfium_test.cc b/samples/pdfium_test.cc
index 417bd56..aa171af 100644
--- a/samples/pdfium_test.cc
+++ b/samples/pdfium_test.cc
@@ -563,8 +563,13 @@
   }
 
   FORM_DoDocumentAAction(form, FPDFDOC_AACTION_WC);
+
+  // Note: The shut down order here is the reverse of the non-XFA branch order.
+  // Need to work out if this is required, and if it is, the lifetimes of
+  // objects owned by |doc| that |form| reference.
   FPDF_CloseDocument(doc);
   FPDFDOC_ExitFormFillEnvironment(form);
+
   FPDFAvail_Destroy(pdf_avail);
 
   fprintf(stderr, "Rendered %d pages.\n", rendered_pages);