DOC: Improving docstring of reset_index method (#16416)
 - [ ] closes #xxxx
 - [ ] tests added / passed
 - [ ] passes ``git diff upstream/master -u -- "*.py" | flake8 --diff``
 - [ ] whatsnew entry

I failed to see that someone else was taking care of the reset_index documentation. But I think the examples I implemented are more clear, so submitting the PR anyway.