Clarify documented semantics of StatefulWidget (#12880)

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

@@ -692,7 +692,7 @@ abstract class StatelessWidget extends Widget {
 /// location). The [State] objects associated with [StatefulWidget] are grafted
 /// along with the rest of the subtree, which means the [State] object is reused
 /// (instead of being recreated) in the new location. However, in order to be
-/// eligible for grafting, the widget might be inserted into the new location in
+/// eligible for grafting, the widget must be inserted into the new location in
 /// the same animation frame in which it was removed from the old location.
 ///
 /// ## Performance considerations