Description of pandas_datetime_exec function.
Added the function description documentation for `pandas_datetime_exec` in the following location.

https://github.com/pandas-dev/pandas/blob/main/pandas/_libs/src/datetime/pd_datetime.c

- [x] closes #61631 (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.

@mroeschke Can you please review this change whenever you get a chance? Thanks @kunaljani1100  Thanks you very much @mroeschke for taking a look into this.