BUG: Add note in whatsnew for DataFrame.at behavior change
- [x] closes #48296 (Replace xxxx with the Github issue number)
- [ ] [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
- [ ] 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.
- [x] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.

The issue mentioned above is just for tracking the change in behavior in 1.5. I've opened #48323 to track the remaining behavior issue mentioned there.
Do you know if we have a test for the above behavior change? Couldn't find one at first glance Ah, should have thought of that; thanks @mroeschke - one on the way. thx @rhshadrach 