Add UniqueKey documentation regarding const constructor (#46647)

packages/flutter/lib/src/widgets/framework.dart

@@ -39,8 +39,15 @@ export 'package:flutter/rendering.dart' show RenderObject, RenderBox, debugDumpR
 // KEYS
 
 /// A key that is only equal to itself.
+///
+/// This cannot be created with a const constructor because that implies that
+/// all instantiated keys would be the same instance and therefore not be unique.
 class UniqueKey extends LocalKey {
   /// Creates a key that is equal only to itself.
+  ///
+  /// The key cannot be created with a const constructor because that implies
+  /// that all instantiated keys would be the same instance and therefore not
+  /// be unique.
   // ignore: prefer_const_constructors_in_immutables , never use const for this class
   UniqueKey();