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/workspace/src/item.rs

@@ -60,7 +60,13 @@ impl ClosePosition {
 
 #[derive(Clone, Default, Serialize, Deserialize, JsonSchema)]
 pub struct ItemSettingsContent {
+    /// Whether to show the Git file status on a tab item.
+    ///
+    /// Default: true
     git_status: Option<bool>,
+    /// Position of the close button in a tab.
+    ///
+    /// Default: right
     close_position: Option<ClosePosition>,
 }