DOC GH22897 Fix docstring of join in pandas/core/frame.py
- [X] closes #22897
- [X] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

#22900 was dealt with by adding an "import pandas as pd" line to the start of the examples section.