[MINOR][SQL] Fix wrong comments on org.apache.spark.sql.parquet.row.a…

…ttributes

## What changes were proposed in this pull request?

This PR fixes the wrong comment on `org.apache.spark.sql.parquet.row.attributes`
which is useful for UDTs like Vector/Matrix. Please see [SPARK-22320](https://issues.apache.org/jira/browse/SPARK-22320) for the usage.

Originally, [SPARK-19411](bf49368#diff-ee26d4c4be21e92e92a02e9f16dbc285L314) left this behind during removing optional column metadatas. In the same PR, the same comment was removed at line 310-311.

## How was this patch tested?

N/A (This is about comments).

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #20346 from dongjoon-hyun/minor_comment_parquet.

sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/parquet/ParquetFileFormat.scala

@@ -108,8 +108,7 @@ class ParquetFileFormat
 
     ParquetOutputFormat.setWriteSupportClass(job, classOf[ParquetWriteSupport])
 
-    // We want to clear this temporary metadata from saving into Parquet file.
-    // This metadata is only useful for detecting optional columns when pushdowning filters.
+    // This metadata is useful for keeping UDTs like Vector/Matrix.
     ParquetWriteSupport.setSchema(dataSchema, conf)
 
     // Sets flags for `ParquetWriteSupport`, which converts Catalyst schema to Parquet