[SPARK-44421][CONNECT][FOLLOWUP] Minor comment improvements

### What changes were proposed in this pull request?

Improve some comments about iterator retries.

### Why are the changes needed?

Improve comments based on followup questions.

### Does this PR introduce _any_ user-facing change?

No.

### How was this patch tested?

No code changes, only comment changes.

Closes apache#42281 from juliuszsompolski/SPARK-44624-comment-only.

Lead-authored-by: Juliusz Sompolski <julek@databricks.com>
Co-authored-by: Hyukjin Kwon <gurwls223@apache.org>
Signed-off-by: Hyukjin Kwon <gurwls223@apache.org>

connector/connect/client/jvm/src/main/scala/org/apache/spark/sql/connect/client/ExecutePlanResponseReattachableIterator.scala

@@ -40,8 +40,13 @@ import org.apache.spark.internal.Logging
  * ExecutePlanResponse on the iterator to return a new iterator from server that continues after
  * that.
  *
- * Since in reattachable execute the server does buffer some responses in case the client needs to
- * backtrack
+ * In reattachable execute the server does buffer some responses in case the client needs to
+ * backtrack. To let server release this buffer sooner, this iterator asynchronously sends
+ * ReleaseExecute RPCs that instruct the server to release responses that it already processed.
+ *
+ * Note: If the initial ExecutePlan did not even reach the server and execution didn't start, the
+ * ReattachExecute can still fail with INVALID_HANDLE.OPERATION_NOT_FOUND, failing the whole
+ * operation.
  */
 class ExecutePlanResponseReattachableIterator(
     request: proto.ExecutePlanRequest,
@@ -86,6 +91,8 @@ class ExecutePlanResponseReattachableIterator(
   private var responseComplete: Boolean = false
 
   // Initial iterator comes from ExecutePlan request.
+  // Note: This is not retried, because no error would ever be thrown here, and GRPC will only
+  // throw error on first iterator.hasNext() or iterator.next()
   private var iterator: java.util.Iterator[proto.ExecutePlanResponse] =
     rawBlockingStub.executePlan(initialRequest)
 

connector/connect/client/jvm/src/main/scala/org/apache/spark/sql/connect/client/GrpcRetryHandler.scala

@@ -99,6 +99,9 @@ private[client] class GrpcRetryHandler(private val retryPolicy: GrpcRetryHandler
       extends StreamObserver[U] {
 
     private var opened = false // only retries on first call
+
+    // Note: This is not retried, because no error would ever be thrown here, and GRPC will only
+    // throw error on first iterator.hasNext() or iterator.next()
     private var streamObserver = call(request)
 
     override def onNext(v: U): Unit = {