DOC: fix DataFrame.replace and DataFrame.set_index docstrings
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

Based on #22459. Fix the docstring for DataFrame.replace and DataFrame.set_index. I put them together since set_index was quite simple: there was two Returns sections.

I also updated ci/code_checks.sh. DataFrame.stack was already correct.
