DOC: Extend docstring pandas core index to_frame method
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

This PR provides adaptations of the original `to_frame` docstring, taking into account the [docstring guide](https://python-sprints.github.io/pandas/guide/pandas_docstring.html). 
