We should mention the explain output changes (related to spilling) for $lookup pushed down into SBE.

Description of Linked Ticket

The owner of this ticket should evaluate the current set of exec stats that the classic DocumentSourceLookup exposes and determine if that set of metrics makes sense in sbe. 

If so we should support those metrics, or alternatively we should come up with a set of metrics that will be useful for the user when running queries with $lookup in sbe.