DOC: Add comment explaining ADV NaN test expected result calculation.
DOC: Add comment explaining ADV NaN test expected result calculation.

bartosh/zipline,magne-max/zipline-ja,humdings/zipline,florentchandelier/zipline,quantopian/zipline,quantopian/zipline,florentchandelier/zipline,Scapogo/zipline,enigmampc/catalyst,alphaBenj/zipline,humdings/zipline,bartosh/zipline,magne-max/zipline-ja,enigmampc/catalyst,alphaBenj/zipline,Scapogo/zipline