DOC: Fixed documentation for pivot_table margins (#48916)
The documentation for the margins parameter of pivot_table was incorrect. It said that the parameter added rows and columns, but it actually passed them to aggfunc. I used the documentation from the user guide to replace the old documentation, as well as added a sentence to the documentation for aggfunc that explained its role in calculating margins.

- [ ] closes #xxxx (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.
- [ ] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.

Thanks @MatteoRaso  No problem.