DOC: Clarify how AssetExists() is special.
DOC: Clarify how AssetExists() is special.

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