Document auto_update_extensions setting (#58954)

Originally added in https://github.com/zed-industries/zed/pull/46130 but
accidentally dropped in
https://github.com/zed-industries/zed/pull/45276. Bring it back, as
we’ve actually had users ask about this previously and
[recently](https://zed-industries.slack.com/archives/C0AJ37231HS/p1779466189363979),
as well.

Self-Review Checklist:

- [ ] I've reviewed my own diff for quality, security, and reliability
- [ ] Unsafe blocks (if any) have justifying comments
- [ ] The content adheres to Zed's UI standards
([UX/UI](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
and
[icon](https://github.com/zed-industries/zed/blob/main/crates/icons/README.md)
guidelines)
- [ ] Tests cover the new/changed behavior
- [ ] Performance impact has been considered and is acceptable


Release Notes:

- N/A

---------

Co-authored-by: zed-zippy[bot] <234243425+zed-zippy[bot]@users.noreply.github.com>

docs/src/reference/all-settings.md

@@ -192,6 +192,28 @@ Define extensions which should be installed (`true`) or never installed (`false`
 }
 ```
 
+## Auto Update extensions
+
+- Description: Disable auto-updates for specific extensions.
+- Setting: `auto_update_extensions`
+- Default: `{}`
+
+**Options**
+
+By default, every installed extension is auto-updated when Zed starts.
+Add an extension here with `false` to pin it to its currently installed version.
+
+```json [settings]
+{
+  "auto_update_extensions": {
+    "html": false
+  }
+}
+```
+
+Selecting **Install Another Version…** from an extension's `⋯` menu on the Extensions
+page ({#action zed::Extensions}) does this automatically.
+
 ## Autosave
 
 - Description: When to automatically save edited buffers.