Add nanoseconds to timestamps stub file
I also found the documentation to be lacking. I had to try it out to determine if `nanoseconds` would be giving me all nanoseconds until the next full second or just until the next full microsecond (the latter is the case).

- [x] no issue exists to track this (probably small enough)
- [x] tests not applicable, I think
- [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.
- [x] ~~Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.~~ As suggested by @phofl not needed

Thanks @robsdedude 