Update Mistral provider docs following #55443

Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [ ] Unsafe blocks (if any) have justifying comments
- [ ] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [x] Tests **and docs** cover the new/changed behavior
- [ ] Performance impact has been considered and is acceptable

Release Notes:

- N/A or Added/Fixed/Improved ...

Signed-off-by: Gabriel Linder <linder.gabriel@gmail.com>