SessionXML: Add note about `auto` substitution

GUI/Model/Model/SessionXML.h

@@ -26,7 +26,7 @@ class MessageService;
 class QUuid;
 
 namespace GUI::Session::XML {
-
+// NOTE: `auto` will be replaced by `const char* const`
 constexpr auto ItemMimeType = "application/org.bornagainproject.xml.item.z";
 constexpr auto LinkMimeType = "application/org.bornagainproject.fittinglink";
 constexpr auto InstrumentModelTag("InstrumentModel");