Add Grouper Rules discussion to Understanding Grouper

From Shilen Patel: I don't see much about Grouper Rules in this doc [Composite Grouper section of Understanding Grouper]. But I wonder if this particular section should compare composite intersections with this - Grouper rules use case - Composite-ng intersection ?

It seems like it would be good for folks to know when to use one method vs the other depending on their use case.