Quote from this doc page

> The $replaceWith  is an alias for $replaceRoot.

This is actually not true, since the syntax for $replaceRoot is:

{ $replaceRoot: { newRoot: <replacementDocument> } }

whereas the syntax for $replaceWith is :

{ $replaceWith: <replacementDocument> }

I understand why it is named "alias", just I had already some developers taking this literally and failing since they did not add the newRoot part... I like to suggest to point out that this is an alias in terms of the functionality but not in terms of syntax.