DOC: More doc fix for dtype_backend
- [ ] Fix all changed in #54104, following #59021 
- [ ] Closes #59332
- [ ] [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.
- [ ] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.


Honestly, I've lost track of the current PR, but I'll do my utmost to address all comments. If further changes are needed, I'll make them promptly. Thanks @luke396!