Clarify help text for Screen aspect ratio

Add to the help text for the Screen aspect ratio setting that
this setting is only used for the video display and not for the GUI.
The GUI uses the pixel aspect ratio, see commit 1fbdd15

mythtv/programs/mythfrontend/globalsettings.cpp

@@ -2149,6 +2149,7 @@ static HostComboBoxSetting *ScreenAspectRatio()
     gc->addSelection(AppearanceSettings::tr("32:10 (16:10 Side by side)"),    "3.2");
     gc->addSelection(AppearanceSettings::tr("16:20 (16:10 Above and below)"), "0.8");
     gc->setHelpText(AppearanceSettings::tr(
+            "This setting applies to video playback only, not to the GUI. "
             "Most modern displays have square pixels and the aspect ratio of the screen can be "
             "computed from the resolution (default). "
             "The aspect ratio can also be automatically detected from the connected display "