Add "rows" as option to "axis" docstring
The "axis" parameter of `pd.concat()` can take the string `'rows'` as an optional way of explicitly defining the default row axis for concatenation:

```python
import pandas as pd

pd.concat([pd.DataFrame(), pd.DataFrame()], axis="rows")
```

Using `'rows'` instead of `'index'` or `0` might be more intuitive as a counterpart to `'columns'`. In any case, it's a possible option, so it might be helpful to represent that in the docstring.

The proposed update in the docstring uses the suggested format based on pandas docstring guide:
https://pandas.pydata.org/docs/dev/development/contributing_docstring.html#parameter-types

>For axis, the convention is to use something like:
>axis : {0 or ‘index’, 1 or ‘columns’, None}, default None

- [ ] closes #xxxx (Replace xxxx with the Github issue number)
- [ ] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature
- [ ] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [ ] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.

Thanks for the pull request but it appears that this change is unlikely to move forward with our pervasive usage of "index". Closing