DOC: fix DataFrame.sample doctests and reformat the docstring
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

Based on #22459. Fix the docstring for DataFrame.sample. I also updated `ci/doctests.sh`.

I was on the fence if I should remove mentions of `Panels` since it is deprecated.