Add a URL for the Qt retina markup bug.

Change-Id: Ie6f3e11a1637f273a9e8e8ad61a97af628d67302
Reviewed-on: https://code.wireshark.org/review/11034
Reviewed-by: Gerald Combs <gerald@wireshark.org>

ui/qt/main_status_bar.cpp

@@ -203,6 +203,7 @@ void MainStatusBar::captureFileClosing() {
 
 void MainStatusBar::expertUpdate() {
     // <img> won't load @2x versions in Qt versions earlier than 5.4.
+    // https://bugreports.qt.io/browse/QTBUG-36383
     // We might have to switch to a QPushButton.
     QString img_text = "<a href><img src=\":/expert/expert_";
     QString tt_text = tr(" is the highest expert information level");