Bug 1175860 - Add some documentation to UploadLastDir to make its workings clearer. r=baku

dom/html/HTMLInputElement.cpp

@@ -805,6 +805,9 @@ UploadLastDir::StoreLastUsedDirectory(nsIDocument* aDoc, nsIFile* aDir)
     return NS_ERROR_OUT_OF_MEMORY;
   prefValue->SetAsAString(unicodePath);
 
+  // Use the document's current load context to ensure that the content pref
+  // service doesn't persistently store this directory for this domain if the
+  // user is using private browsing:
   nsCOMPtr<nsILoadContext> loadContext = aDoc->GetLoadContext();
   return contentPrefService->Set(spec, CPS_PREF_NAME, prefValue, loadContext, nullptr);
 }