[MINOR] Remove InstantGeneratorOperator parallelism limit in HoodieFlinkStreamer and update docs (#2471)

hudi-flink/src/main/java/org/apache/hudi/HoodieFlinkStreamer.java

@@ -93,11 +93,10 @@ public class HoodieFlinkStreamer {
             .name("kafka_to_hudi_record")
             .uid("kafka_to_hudi_record_uid");
 
-    // InstantGenerateOperator helps to emit globally unique instantTime, it must be executed in one parallelism
+    // InstantGenerateOperator helps to emit globally unique instantTime
     inputRecords.transform(InstantGenerateOperator.NAME, TypeInformation.of(HoodieRecord.class), new InstantGenerateOperator())
         .name("instant_generator")
         .uid("instant_generator_id")
-        .setParallelism(1)
 
         // Keyby partition path, to avoid multiple subtasks writing to a partition at the same time
         .keyBy(HoodieRecord::getPartitionPath)

hudi-flink/src/main/java/org/apache/hudi/operator/InstantGenerateOperator.java

@@ -57,10 +57,9 @@ import java.util.concurrent.TimeUnit;
 import java.util.concurrent.atomic.AtomicLong;
 
 /**
- * Operator helps to generate globally unique instant, it must be executed in one parallelism. Before generate a new
+ * Operator helps to generate globally unique instant. Before generate a new instant {@link InstantGenerateOperator}
- * instant , {@link InstantGenerateOperator} will always check whether the last instant has completed. if it is
+ * will always check whether the last instant has completed. if it is completed and has records flows in, a new instant
- * completed, a new instant will be generated immediately, otherwise, wait and check the state of last instant until
+ * will be generated immediately, otherwise, wait and check the state of last instant until time out and throw an exception.
- * time out and throw an exception.
  */
 public class InstantGenerateOperator extends AbstractStreamOperator<HoodieRecord> implements OneInputStreamOperator<HoodieRecord, HoodieRecord> {
 
@@ -128,11 +127,11 @@ public class InstantGenerateOperator extends AbstractStreamOperator<HoodieRecord
     super.prepareSnapshotPreBarrier(checkpointId);
     String instantMarkerFileName = String.format("%d%s%d%s%d", indexOfThisSubtask, DELIMITER, checkpointId, DELIMITER, recordCounter.get());
     Path path = new Path(new Path(HoodieTableMetaClient.AUXILIARYFOLDER_NAME, INSTANT_MARKER_FOLDER_NAME), instantMarkerFileName);
-    // mk marker file by each subtask
+    // create marker file
     fs.create(path, true);
     LOG.info("Subtask [{}] at checkpoint [{}] created marker file [{}]", indexOfThisSubtask, checkpointId, instantMarkerFileName);
     if (isMain) {
-      // check whether the last instant is completed, if not, wait 10s and then throws an exception
+      // check whether the last instant is completed, will try specific times until an exception is thrown
       if (!StringUtils.isNullOrEmpty(latestInstant)) {
         doCheck();
         // last instant completed, set it empty
@@ -264,7 +263,7 @@ public class InstantGenerateOperator extends AbstractStreamOperator<HoodieRecord
     }
 
     boolean receivedData = false;
-    // judge whether has data in this checkpoint and delete maker file.
+    // check whether has data in this checkpoint and delete maker file.
     for (FileStatus fileStatus : fileStatuses) {
       Path path = fileStatus.getPath();
       String name = path.getName();