docs: Add missing font fallback documentation (#55779)

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

docs/src/reference/all-settings.md

@@ -4500,6 +4500,24 @@ The name of any font family installed on the user's system
 }
 ```
 
+### Terminal: Font Fallbacks
+
+- Description: The font fallbacks to use for text in the terminal. Uses the buffer's font fallback if unset.
+- Setting: "font_fallbacks"
+- Default: `null`
+
+**Options**
+
+For example, to use Nerd Font as a fallback, add the following to your settings:
+
+```json [settings]
+{
+  "terminal": {
+    "font_fallbacks": ["Nerd Font"]
+  }
+}
+```
+
 ### Terminal: Font Features
 
 - Description: What font features to use for the terminal. When not set, defaults to matching the editor's font features.