Comments about the purpose of SettingName for radio buttons

src/ShuttleGui.h

@@ -193,6 +193,7 @@ public:
    wxPanel * StartInvisiblePanel();
    void EndInvisiblePanel();
 
+   // SettingName is a key in Preferences.
    void StartRadioButtonGroup( const ChoiceSetting &Setting );
    void EndRadioButtonGroup();