[macOS] Document onKeyboardLayoutChanged (flutter/engine#33663)

Replaces a TODO with documentation. No tests changed, since this change
simply adds a missing doc comment.

engine/src/flutter/shell/platform/darwin/macos/framework/Source/FlutterViewController.mm

@@ -220,7 +220,12 @@ NSData* currentKeyboardLayoutData() {
  */
 - (void)dispatchMouseEvent:(nonnull NSEvent*)event phase:(FlutterPointerPhase)phase;
 
-// TODO
+/**
+ * Called when the active keyboard input source changes.
+ *
+ * Input sources may be simple keyboard layouts, or more complex input methods involving an IME,
+ * such as Chinese, Japanese, and Korean.
+ */
 - (void)onKeyboardLayoutChanged;
 
 @end