API: read_csv, to_csv line_terminator keyword inconsistency
- [x] closes #9568
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry (pending 1.2)

I followed discussion in #9568. The aim of this PR is to allow both `line_terminator` and `lineterminator` keyword args  to `read_csv` and `to_csv` but only document `line_terminator`. As per @jorisvandenbossche's suggestion we preserve `lineterminator` to stay compatible with `csv` dialects.
