Final: Add documentation and migration guidance for unification

Co-authored-by: kateinoigakukun <[email protected]>

Author: Copilot

Sources/JavaScriptKit/FundamentalObjects/JSClosure.swift

@@ -103,6 +103,9 @@ public class JSOneshotClosure: JSObject, JSClosureProtocol {
 /// button.removeEventListener!("click", JSValue.function(eventListener))
 /// ```
 ///
+/// - Note: JSClosure now inherits from JSObject directly instead of JSFunction,
+///   providing a consistent and unified object model that aligns with JavaScript's
+///   dynamic callability semantics.
 public class JSClosure: JSObject, JSClosureProtocol {
 
     class SharedJSClosure {

Sources/JavaScriptKit/FundamentalObjects/JSFunction.swift

@@ -10,6 +10,9 @@ import _CJavaScriptKit
 /// alert("Hello, world")
 /// ```
 ///
+/// - Note: In a future version, JSFunction will be unified with JSObject.
+///   Consider using JSObject directly for new code, as all objects in JavaScript
+///   can potentially be callable. JSFunction functionality is now available on JSObject.
 public class JSFunction: JSObject {
     #if !hasFeature(Embedded)
     /// Call this function with given `arguments` and binding given `this` as context.