[Python][Docs] Update Python docstrings to reflect new parquet encoding option

### Describe the enhancement requested

Docstrings have not kept up with [encoder/decoder capabilities](https://github.com/rok/arrow/blob/5039cf9ae7b15bc3fe8c7741f520c27c98076224/docs/source/cpp/parquet.rst#encodings).
See [comment](https://github.com/apache/arrow/pull/14341#issuecomment-1687879350) for context.

### Component(s)

Python