Update docs/en/reference/batch-processing.rst

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.
Merge pull request #576 from acasademont/patch-1

Update docs/en/reference/batch-processing.rst