Display setting documentation in settings.json (#3936)

Let this screenshot of settings.json speak for itself: 

![image](https://github.com/zed-industries/zed/assets/24362066/fca60383-1788-43f9-803b-00f083394c8a)

Release Notes:
- Added code completion & on-hover documentation to Zed's settings.json
file.

---------

Co-authored-by: Marshall Bowers <elliott.codes@gmail.com>

crates/auto_update/src/auto_update.rs

@@ -10,6 +10,7 @@ use gpui::{
 };
 use isahc::AsyncBody;
 
+use schemars::JsonSchema;
 use serde::Deserialize;
 use serde_derive::Serialize;
 use smol::io::AsyncReadExt;
@@ -61,18 +62,27 @@ struct JsonRelease {
 
 struct AutoUpdateSetting(bool);
 
+/// Whether or not to automatically check for updates.
+///
+/// Default: true
+#[derive(Clone, Default, JsonSchema, Deserialize, Serialize)]
+#[serde(transparent)]
+struct AutoUpdateSettingOverride(Option<bool>);
+
 impl Settings for AutoUpdateSetting {
     const KEY: Option<&'static str> = Some("auto_update");
 
-    type FileContent = Option<bool>;
+    type FileContent = AutoUpdateSettingOverride;
 
     fn load(
-        default_value: &Option<bool>,
-        user_values: &[&Option<bool>],
+        default_value: &Self::FileContent,
+        user_values: &[&Self::FileContent],
         _: &mut AppContext,
     ) -> Result<Self> {
         Ok(Self(
-            Self::json_merge(default_value, user_values)?.ok_or_else(Self::missing_default)?,
+            Self::json_merge(default_value, user_values)?
+                .0
+                .ok_or_else(Self::missing_default)?,
         ))
     }
 }