Adds missing documentation about font fallbacks on `appearance.md` and
`all-settings.md`
`all-settings.md` only lacked terminal font fallback documentation so
that's the only thing I added there.

Self-Review Checklist:

- [X] I've reviewed my own diff for quality, security, and reliability
- [it's a doc update] Unsafe blocks (if any) have justifying comments
- [there is no ui] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [it's a doc update] Tests cover the new/changed behavior
- [i really hope that my extra bytes on documentation cause no
performance impacts] Performance impact has been considered and is
acceptable

Release Notes:
- N/A