DOC: add plt.show so that the plot will show, along with note about p…
…lt.show, plt.savefig and Jupyter Notebook.

- [x] closes #43560
- [ ] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit) for how to run them
- [ ] whatsnew entry
