Add comment + test for slider multiplier

osu.Game/Rulesets/UI/Scrolling/DrawableScrollingRuleset.cs

@@ -131,6 +131,8 @@ namespace osu.Game.Rulesets.UI.Scrolling
                     if (duration > maxDuration)
                     {
                         maxDuration = duration;
+                        // The slider multiplier is post-multiplied to determine the final velocity, but for relative scale beat lengths
+                        // the multiplier should not affect the effective timing point (the longest in the beatmap), so it is factored out here
                         baseBeatLength = timingPoints[i].BeatLength / Beatmap.BeatmapInfo.BaseDifficulty.SliderMultiplier;
                     }
                 }