Document optional kwargs argument in Partition Ptransform

All PTransforms that accept a callable accept kwargs that will be passed to the callable.
Some PTransfoms document that in their docstring, for example, see Map[1].

It looks like Partition transform does not document kwargs in docstring, althougth it also accepts them [2].

[1] https://github.com/apache/beam/blob/05900a43de1f380f124c688cd8d804b5133bda64/sdks/python/apache_beam/transforms/core.py#L1277

[2] https://github.com/apache/beam/blob/05900a43de1f380f124c688cd8d804b5133bda64/sdks/python/apache_beam/transforms/core.py#L1934

Imported from Jira [BEAM-7671](https://issues.apache.org/jira/browse/BEAM-7671). Original Jira may contain additional context.
Reported by: tvalentyn.