DOC: Clarify when csv separator is being parsed as regex. Resolves #10208
- [x] closes #10208
- [ ] tests added / passed -- only docs, not needed
- [x] passes `git diff upstream/master | flake8 --diff`
- [ ] whatsnew entry -- neccessary for docs?

also update the docs in `io.rst` (with the same), though you can be 
 Updated!
 thanks!
