This ticket allows the projection field provided to the find() command to contain aggregation expressions and the aggregation "nested object syntax" (e.g. {a: {b: 1}} is allowed in addition to {"a.b": 1}).
 

This ticket also changed the 'fields' parameter of findAndModify() to accept the same syntax.

I believe this page will need to be updated:
https://docs.mongodb.com/manual/reference/method/db.collection.find/#find-projection

And this too:
https://docs.mongodb.com/manual/reference/method/db.collection.findAndModify/