Add comments for the optional null check.

Signed-off-by: Rishikesh1159 <rishireddy1159@gmail.com>

server/src/test/java/org/opensearch/index/ShardIndexingPressureConcurrentExecutionTests.java

@@ -269,6 +269,8 @@ public void testCoordinatingPrimaryThreadedUpdateToShardLimitsAndRejections() th
 
         nodeStats = shardIndexingPressure.stats();
         IndexingPressurePerShardStats shardStoreStats = shardIndexingPressure.shardStats().getIndexingPressureShardStats(shardId1);
+        // If rejection count equals NUM_THREADS that means rejections happened until the last request, then we'll get shardStoreStats which
+        // was updated on the last request. In other cases, the shardStoreStats simply moves to the cold store and null is returned.
         if (rejectionCount.get() == NUM_THREADS) {
             assertEquals(10, shardStoreStats.getCurrentPrimaryAndCoordinatingLimits());
         } else {
@@ -335,6 +337,8 @@ public void testReplicaThreadedUpdateToShardLimitsAndRejections() throws Excepti
         assertEquals(0, nodeStats.getCurrentReplicaBytes());
 
         IndexingPressurePerShardStats shardStoreStats = shardIndexingPressure.shardStats().getIndexingPressureShardStats(shardId1);
+        // If rejection count equals NUM_THREADS that means rejections happened until the last request, then we'll get shardStoreStats which
+        // was updated on the last request. In other cases, the shardStoreStats simply moves to the cold store and null is returned.
         if (rejectionCount.get() == NUM_THREADS) {
             assertEquals(15, shardStoreStats.getCurrentReplicaLimits());
         } else {