Add explanation for #1587 changes

src/core/PresetPreviewPlayHandle.cpp

@@ -147,6 +147,8 @@ PresetPreviewPlayHandle::PresetPreviewPlayHandle( const QString & _preset_file,
 	else
 	{
 		DataFile dataFile( _preset_file );
+		// vestige previews are bug prone; fallback on 3xosc with volume of 0
+		// without an instrument in preview track, it will segfault
 		if(dataFile.content().elementsByTagName( "vestige" ).length() == 0 )
 		{
 			s_previewTC->previewInstrumentTrack()->