Add docs

Author: intitni

Tool/Sources/BuiltinExtension/BuiltinExtension.swift

@@ -1,7 +1,13 @@
 import CopilotForXcodeKit
 import Foundation
+import Preferences
 
 public protocol BuiltinExtension: CopilotForXcodeExtensionCapability {
+    /// An id that let the extension manager determine whether the extension is in use.
+    var suggestionServiceId: BuiltInSuggestionFeatureProvider { get }
+
+    /// It's usually called when the app is about to quit,
+    /// you should clean up all the resources here.
     func terminate()
 }