a stage operator that will unwind a subdocument so that a copy of the parent document is output along with each element of the subdocument - project every element of the subdocument to its own copy of the parent document. downstream pipeline stage operations may operate on the output akin to unwind.

NOTES on Priority: I currently work around this haphazardly if I can deterministically know in advance all of the elements that are in the subdocument.

NOTES on Affected Version: Not sure what to put here for this.