[Java][Docs] Document the use of the batchSize argument in Dataset ScanOptions

Several ScanOptions methods take a batchSize argument as shown: 

`public ScanOptions(long batchSize) {`
`    this(batchSize, Optional.empty());`
``}

Since the scanner reads one ArrowRecordBatch per load invocation, setting the parameter to a size larger than the RecordBatch has no effect. It only works when it's smaller than the number of rows in the RecordBatch, (i.e., the number or records read is equal to min(batchSize, recordBatch rowCount), potentially leading to some confusion. 

**Reporter**: [Larry White](https://issues.apache.org/jira/browse/ARROW-17346) / @lwhite1

_{**Note**: *This issue was originally created as [ARROW-17346](https://issues.apache.org/jira/browse/ARROW-17346). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}

batchSizeを受け取るDataset ScanOptionsのメソッドから始め、スキャナーが呼び出しごとに1つのArrowRecordBatchを読み込む仕組みを追跡します。RecordBatchの行数より大きい値には追加の効果がなく、小さい値ではレコード数がmin(batchSize, recordBatch rowCount)に制限されることを文書化します。ScanOptionsのドキュメントでこの動作が明確になれば完了です。

索引モデルが issue の本文から書いたものです。