[Python][Doc] Documentation for Test Groups out of sync

In the Python documentation for [Test Groups](https://arrow.apache.org/docs/dev/developers/python.html#test-groups) it's stated:
> We have many tests that are grouped together using pytest marks. Some of these are disabled by default. To enable a test group, pass --$GROUP_NAME, e.g. --parquet. [continues]
However, I get:

`$ pytest --parquet # from arrow/python dir`
`ERROR: usage: pytest [options] [file_or_dir] [file_or_dir] [...]`
`pytest: error: unrecognized arguments: --parquet`

Am I interpreting the docs wrong or has something changed since writing the docs?

Related: `pytest --markers` only reports the full list of pytest Marks when I'm in the `arrow/python/pyarrow` directory but not `arrow/python`. When I'm in the `arrow/python/pyarrow` subdirectory, marks work as expected (ie `pytest -m parquet works`).

**Reporter**: [Bryce Mecum](https://issues.apache.org/jira/browse/ARROW-18066) / @amoeba

_{**Note**: *This issue was originally created as [ARROW-18066](https://issues.apache.org/jira/browse/ARROW-18066). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}

Start with the Python documentation's Test Groups section and reproduce the documented pytest --parquet command from arrow/python, then compare it with pytest --markers and pytest -m parquet from arrow/python/pyarrow. Done means the documentation accurately describes the working command, directory, and marker behavior.

Written by the indexing model from the issue text.