DraggableListBuilder needs to document its expectations

Problem/Motivation

The DraggableListBuilder class is meant to be used to make entity lists, but it requires things which aren't immediately clear.

So far, that seems to be just this one:

- the entity type must declare a weight entity key

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet