DOC: Add details of dropna in DataFrame.pivot_table
- [x] closes #61113 
- [x] [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).
- [x] 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.

Which `doc/source/whatsnew/vX.X.X.rst` file should I edit? `v2.3.0.rst`? ~@rhshadrach it looks like two warnings are raised because of the bulleted list formatting: ([1](https://github.com/pandas-dev/pandas/actions/runs/14161907429/job/39668656803?pr=61184#step:8:61)), ([2](https://github.com/pandas-dev/pandas/actions/runs/14161907429/job/39668656803?pr=61184#step:8:62)). Is there another way you recommend formatting the docstring? Or do you have an example with a bulleted list that I can work from?~

Think I figured it out in 1a82d2a. @rhshadrach ready for review Thanks @it176131 - nice work!