add some comments in ReadSplit

gluten-core/src/main/java/io/glutenproject/substrait/rel/ExtensionTableNode.java

@@ -20,11 +20,10 @@
 import com.google.protobuf.StringValue;
 import io.substrait.proto.ReadRel;
 
-import java.io.Serializable;
 import java.util.ArrayList;
 import java.util.List;
 
-public class ExtensionTableNode implements ReadSplit, Serializable {
+public class ExtensionTableNode implements ReadSplit {
   private static final String MERGE_TREE = "MergeTree;";
   private Long minPartsNum;
   private Long maxPartsNum;

gluten-core/src/main/java/io/glutenproject/substrait/rel/LocalFilesNode.java

@@ -25,12 +25,11 @@
 import org.apache.spark.sql.types.StructField;
 import org.apache.spark.sql.types.StructType;
 
-import java.io.Serializable;
 import java.util.ArrayList;
 import java.util.List;
 import java.util.Map;
 
-public class LocalFilesNode implements ReadSplit, Serializable {
+public class LocalFilesNode implements ReadSplit {
   private final Integer index;
   private final List<String> paths = new ArrayList<>();
   private final List<Long> starts = new ArrayList<>();

gluten-core/src/main/java/io/glutenproject/substrait/rel/ReadSplit.java

@@ -18,9 +18,16 @@
 
 import com.google.protobuf.MessageOrBuilder;
 
+import java.io.Serializable;
 import java.util.List;
 
-public interface ReadSplit {
+/**
+ * A serializable representation of a read split for native engine, including the file path and
+ * other information of the scan table. It is returned by {@link
+ * io.glutenproject.execution.BasicScanExecTransformer#getReadSplits()}.
+ */
+public interface ReadSplit extends Serializable {
+  /** The preferred locations where the table files returned by this read split can run faster. */
   List<String> preferredLocations();
 
   MessageOrBuilder toProtobuf();