add brief documentation for searchLoad

Author: drempapis

server/src/main/java/org/elasticsearch/index/search/stats/SearchStats.java

@@ -49,6 +49,8 @@ public static class Stats implements Writeable, ToXContentFragment {
         private long queryFailure;
         private long fetchFailure;
 
+        //This is the ExponentiallyWeightedMovingRate for a shard, tracking execution time across different phases
+        // (e.g., query, fetch, etc.), favoring more recent load values by assigning them greater significance than older values.
         private double recentSearchLoad;
 
         private Stats() {