Bug 1401069 - Part 8. Give a detailed comment to explain how to react when hitting this assertion in OpenDocument(). r=fatseng

MozReview-Commit-ID: 74UkMV3ZOWm

--HG--
extra : rebase_source : 5fedf61f8a737ecac5668d372bff48b224859fdb

widget/windows/PDFViaEMFPrintHelper.cpp

@@ -37,8 +37,11 @@ nsresult
 PDFViaEMFPrintHelper::OpenDocument(nsIFile *aFile)
 {
   MOZ_ASSERT(aFile);
+
   if (mPDFDoc) {
-    MOZ_ASSERT_UNREACHABLE("We can only open one PDF at a time");
+    MOZ_ASSERT_UNREACHABLE("We can only open one PDF at a time,"
+                           "Use CloseDocument() to close the opened file"
+                           "before calling OpenDocument()");
     return NS_ERROR_FAILURE;
   }