Problem/Motivation
While writing the mongodb content entity storage passing back stored data to ContentEntityBase was the biggest mystery. I have figured it out, I think.
Proposed resolution
Document the array. Add a todo.
Remaining tasks
The whole loadFromMongo method really badly wants to be documentation; this is the first half, up to creating the entity. Where do we put the second half?
Get someone (plach/berdir/fago) to verify the @todo . If correct then #2137801: Refactor entity storage to load field values before instantiating entity objects needs an issue summary update, I think. Ie. I know loadFromMongo works because Drupal\node\Tests\NodeViewLanguageTest and Drupal\node\Tests\NodeFieldMultilingualTest pass since I changed it to do what core does, I think at least -- but the question is, is this the only way?