[GH-576] Update docs/en/reference/batch-processing.rst

This issue is created automatically through a Github pull request on behalf of acasademont:

Url: https://github.com/doctrine/doctrine2/pull/576

Message:

If you have only $batchSize - 1 rows (amongst other cases), the entities are never flushed, you need a final flush outside the loop.

In the bulk insert you should also need a final flush if the number of entities inserted is not a multiple of the $batchSize.