Adds more comments

Author: liancheng

sql/core/src/main/scala/org/apache/spark/sql/parquet/newParquet.scala

@@ -194,6 +194,10 @@ private[sql] class ParquetRelation2(
       committerClass,
       classOf[ParquetOutputCommitter])
 
+    // We're not really using `ParquetOutputFormat[Row]` for writing data here, because we override
+    // it in `ParquetOutputWriter` to support appending and dynamic partitioning.  The reason why
+    // we set it here is to setup the output committer class to `ParquetOutputCommitter`, which is
+    // bundled with `ParquetOutputFormat[Row]`.
     job.setOutputFormatClass(classOf[ParquetOutputFormat[Row]])
 
     // TODO There's no need to use two kinds of WriteSupport