Add XMLDocs to difficulty attribute properties

osu.Game.Rulesets.Catch/Difficulty/CatchDifficultyAttributes.cs

@@ -9,6 +9,12 @@ namespace osu.Game.Rulesets.Catch.Difficulty
 {
     public class CatchDifficultyAttributes : DifficultyAttributes
     {
+        /// <summary>
+        /// The perceived approach rate inclusive of rate-adjusting mods (DT/HT/etc).
+        /// </summary>
+        /// <remarks>
+        /// Rate-adjusting mods don't directly affect the approach rate difficulty value, but have a perceived effect as a result of adjusting audio timing.
+        /// </remarks>
         [JsonProperty("approach_rate")]
         public double ApproachRate { get; set; }