clarify max skia cache size property.

src/Skia/Avalonia.Skia/SkiaOptions.cs

@@ -15,6 +15,8 @@ namespace Avalonia
 
         /// <summary>
         /// The maximum number of bytes for video memory to store textures and resources.
+        /// This is set by default to the recommended value for Avalonia.
+        /// Setting this to null will give you the default Skia value.
         /// </summary>
         public long? MaxGpuResourceSizeBytes { get; set; } = 1024 * 600 * 4 * 12; // ~28mb 12x 1024 x 600 textures. 
     }