Add documentation for negative values of ParallaxAmount

osu.Game/Graphics/Containers/ParallaxContainer.cs

@@ -16,6 +16,9 @@ namespace osu.Game.Graphics.Containers
     {
         public const float DEFAULT_PARALLAX_AMOUNT = 0.02f;
 
+        /// <summary>
+        /// The amount of parallax movement. Negative values will reverse the direction of parallax relative to user input.
+        /// </summary>
         public float ParallaxAmount = DEFAULT_PARALLAX_AMOUNT;
 
         private Bindable<bool> parallaxEnabled;