DOC : fix errors docstrings pandas.to_numeric
- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `black pandas`
- [x ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

added docstrings information about arg descriptions, downcast type 'int', and Return type

Thanks @isoletslicer 