DOC Add documentation for how pandas rounds values in Series.round and Dataframe.round methods
- [ ] closes #xxxx (Replace xxxx with the GitHub issue number)
- [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).
- [ ] 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.

Description: 
In the pandas documentation it is not documented how numbers are rounded if values are exactly halfway between rounded decimal values (they are rounded to the nearest even value). I added the documentation for this behaviour.
Thanks for the review @MarcoGorelli! Until next time! Bye! 
![Alt Text](https://media.giphy.com/media/v1.Y2lkPTc5MGI3NjExNTcxMjhtemtoMnowODNidGFhOGN1Zm5vZHQ0d3JrbmZtazM5MDI0cSZlcD12MV9pbnRlcm5hbF9naWZfYnlfaWQmY3Q9Zw/NqZn5kPN8VVrW/giphy.gif)