Modify documentation for POJO groupBy in - Reduce on DataSet section

Add documentation to changes introduced in [Add support for POJO objects](https://github.com/apache/incubator-flink/commit/3f87fc9f1cc2f0e18552e6ec5e9449228a942726)

- groupBy (DataSet reduce)
- WordCountPOJO

+potentially:
- JoinOperato
- CoGroupOperator