Should we document key for `each` helper in templates?

To do:
- Ask the framework team for some context about whether this is essential info, and how they hope people will use it. @locks is asking on Discord
- @mansona if you have any ideas about why and how to document this, please comment.
- Fill in details for this ticket so someone can work on it

This ticket is not yet ready for a volunteer.

API docs ticket: https://github.com/emberjs/ember.js/issues/19954