Migration Guide: Document that can-map no longer creates a custom can-list

[chasenlehara]

@ccummings commented on Wed Mar 15 2017

In 2.3, creating a custom can.Map would also create a custom can.List whose static Map property was the custom Map. For example:

const MyMap = can.Map.extend({
  myHelper() {
  }
});

//MyMap.List can now be used elsewhere

In 3.0 this was removed.

If the removal is permanent, it should be added to the migration guide.