Add comment for alt pressed

Taken from OsuScrollContainer

osu.Game/Screens/Select/BeatmapCarousel.cs

@@ -412,6 +412,8 @@ void perform()
 
         protected override bool OnKeyDown(KeyDownEvent e)
         {
+            // allow for controlling volume when alt is held.
+            // mostly for compatibility with osu-stable.
             if (e.AltPressed)
                 return base.OnKeyDown(e);