DOC: Resolve RT03 errors in several methods #2 
Resolve all RT03 errors for the following cases:
scripts/validate_docstrings.py --format=actions --errors=RT03 pandas.DataFrame.nsmallest
scripts/validate_docstrings.py --format=actions --errors=RT03 pandas.DataFrame.nunique
scripts/validate_docstrings.py --format=actions --errors=RT03 pandas.DataFrame.pipe
scripts/validate_docstrings.py --format=actions --errors=RT03 pandas.DataFrame.plot.box
scripts/validate_docstrings.py --format=actions --errors=RT03 pandas.DataFrame.plot.density
scripts/validate_docstrings.py --format=actions --errors=RT03 pandas.DataFrame.plot.kde
scripts/validate_docstrings.py --format=actions --errors=RT03 pandas.DataFrame.plot.scatter

- xref DOC: fix RT03 errors in docstrings  #57416
- [ ] [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.

Thanks @bergnerjonas 