Update MaterialStatesController docs for calling setState in a li…

…stener (#143453)

fixes [Calling `setState` in a `MaterialStatesController` listener and `MaterialStateController.update` causes Exception](flutter/flutter#138986)

### Description
`MaterialStatesController` listener  calls `setState` during build when `MaterialStatesController.update` listener calls `notifyListeners`.

I tried fixing this issue by putting `notifyListeners` in a post-frame callback. However, this breaks existing customer tests (particularly super editor tests).

A safer approach would be to document that the listener's `setState` call should be in a post-frame callback to delay it and not call this during the build phase triggered by the `MaterialStatesController.update` in the widgets such as InkWell or buttons.

packages/flutter/lib/src/material/material_state.dart

@@ -760,6 +760,14 @@ class MaterialStatePropertyAll<T> implements MaterialStateProperty<T> {
 /// [MaterialState.focused] to its controller. When the widget gains the
 /// or loses the focus it will [update] its controller's [value] and
 /// notify listeners of the change.
+///
+/// When calling `setState` in a [MaterialStatesController] listener, use the
+/// [SchedulerBinding.addPostFrameCallback] to delay the call to `setState` after
+/// the frame has been rendered. It's generally prudent to use the
+/// [SchedulerBinding.addPostFrameCallback] because some of the widgets that
+/// depend on [MaterialStatesController] may call [update] in their build method.
+/// In such cases, listener's that call `setState` - during the build phase - will cause
+/// an error.
 class MaterialStatesController extends ValueNotifier<Set<MaterialState>> {
   /// Creates a MaterialStatesController.
   MaterialStatesController([Set<MaterialState>? value]) : super(<MaterialState>{...?value});