Add comment explaining the purpose of the empty FilterTerms

osu.Game/Overlays/Settings/Sections/Input/KeyBindingsSubsection.cs

@@ -76,6 +76,7 @@ namespace osu.Game.Overlays.Settings.Sections.Input
             Content.CornerRadius = 5;
         }
 
+        // Empty FilterTerms so that the ResetButton is visible only when the whole subsection is visible.
         public override IEnumerable<string> FilterTerms => Enumerable.Empty<string>();
     }
 }