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/diagnostics/src/project_diagnostics_settings.rs

@@ -6,8 +6,12 @@ pub struct ProjectDiagnosticsSettings {
     pub include_warnings: bool,
 }
 
+/// Diagnostics configuration.
 #[derive(Clone, Default, Serialize, Deserialize, JsonSchema, Debug)]
 pub struct ProjectDiagnosticsSettingsContent {
+    /// Whether to show warnings or not by default.
+    ///
+    /// Default: true
     include_warnings: Option<bool>,
 }