Use super to propagate processLumino/PhosphorMessage instead of skipping to the top of the prototype hierarchy immediately, in the migration guide.

The code before would jump straight to the root class, rather than going up the hierarchy.

Author: jasongrout

docs/source/migration_guides.md

@@ -88,7 +88,7 @@ The ``DOMWidgetView.pWidget`` property has been renamed ``DOMWidgetView.luminoWi
 + this.luminoWidget
 ```
 
-The ``DOMWidgetView.processPhosphorMessage`` method has been renamed ``DOMWidgetView.processLuminoMessage``. If you want to support both ipywidgets 7.x and 8.x, you should implement both methods:
+The ``DOMWidgetView.processPhosphorMessage`` method has been renamed ``DOMWidgetView.processLuminoMessage``. If you want to support both ipywidgets 7.x and 8.x, you should implement both methods and call the correct super method:
 
 ```diff
 - processPhosphorMessage(msg: Message): void {
@@ -109,15 +109,15 @@ The ``DOMWidgetView.processPhosphorMessage`` method has been renamed ``DOMWidget
 + }
 +
 + processPhosphorMessage(msg: Message): void {
-+     this._processLuminoMessage(msg, (DOMWidgetView as any).processPhosphorMessage);
++     this._processLuminoMessage(msg, super.processPhosphorMessage);
 + }
 +
 + processLuminoMessage(msg: Message): void {
-+     this._processLuminoMessage(msg, (DOMWidgetView as any).processLuminoMessage);
++     this._processLuminoMessage(msg, super.processLuminoMessage);
 + }
 ```
 
-If you're dropping ipywidgets 7.x support, you can simply rename the `processPhosphorMessage` method into `processLuminoMessage`.
+If you're dropping ipywidgets 7.x support, you can simply rename the `processPhosphorMessage` method to `processLuminoMessage`.
 
 #### Widget manager import