Add xmldoc explaining how PreviewTime is intended to work

osu.Game/Beatmaps/BeatmapMetadata.cs

@@ -51,7 +51,12 @@ namespace osu.Game.Beatmaps
         [JsonProperty(@"tags")]
         public string Tags { get; set; }
 
+        /// <summary>
+        /// The time in milliseconds to begin playing the track for preview purposes.
+        /// If -1, the track should begin playing at 40% of its length.
+        /// </summary>
         public int PreviewTime { get; set; }
+
         public string AudioFile { get; set; }
         public string BackgroundFile { get; set; }