DOC: Document merge_cells="columns" in to_excel

[rhshadrach]

https://pandas.pydata.org/docs/dev/reference/api/pandas.DataFrame.to_excel.html

The merge_cells argument can also take "columns" due to #35384. This should be added to the docstring.

[ZKaoChi]

take

[ZKaoChi]

I found it was added once, but then overwritten.