DeleteStage handles WriteConflictExceptions/TemporarilyUnavailableExceptions when restoring its children state by returning NEED_YIELD. If the stage needs to return the deleted document, it annotates it on `_idReturning` so that on the next `doWork` it is returned. However, if the delete was "justOne: true", the next `doWork` will immediately return EOF instead of the pending document.

If this happened, findAndModify would delete the document but not return it.