DOC: complete parameter descriptions for DataFrame.insert's docstring
issue: https://github.com/jorisvandenbossche/pydata-amsterdam-pandas-sprint/issues/1
- bullet point item #5: "complete the parameter descriptions"

- [x] closes https://github.com/jorisvandenbossche/pydata-amsterdam-pandas-sprint/issues/1, bullet point 5
- [x] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).

@jorisvandenbossche 

Thanks @mcnoat 