XFA: Add a comment regarding the shut down order.

samples/pdfium_test.cc

Index: samples/pdfium_test.cc
diff --git a/samples/pdfium_test.cc b/samples/pdfium_test.cc
index 417bd5603b229528130daea57ca64c8e20e94b97..aa171afe3f212460e7d53d6e2bc953569a0ea1ef 100644
--- a/samples/pdfium_test.cc
+++ b/samples/pdfium_test.cc
@@ -563,8 +563,13 @@ void RenderPdf(const std::string& name, const char* pBuf, size_t len,
   }
 
   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);