add warning about setting max_rows/max_columns to 'None'
[I teach pandas to students who are new to coding](https://python-public-policy.afeld.me/en/nyu/syllabus.html), and [when I introduce them to `display.max_rows`](https://python-public-policy.afeld.me/en/nyu/lecture_1.html#excluding-bad-records-from-the-dataframe), their instinct is (reasonably) "why can't I just display everything?" Adding this caution to hopefully help clarify.

- [ ] ~~closes #xxxx~~
- [ ] ~~[Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature~~
- [x] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [ ] ~~Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.~~
- [ ] ~~Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.~~

Thanks @afeld 