DOC: NDFrame.to_hdf(data_columns) documented (#13061).
- [x] closes #13061
- [ ] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [ ] whatsnew entry

I'm aware there's another PR for the same issue, but it hasn't had activity in a while so this is my attempt.  I took the opportunity to remove the unusable "mode=r" option from the same docstring, and make the title clearer.
