Added documentation for skip, take, lastOrNil and firstOrNil
Added documentation for skip, take, lastOrNil and firstOrNil

ShengQiangLiu/LinqToObjectiveC,ChetanGandhi/LinqToObjectiveC,suxinde2009/LinqToObjectiveC,ShengQiangLiu/LinqToObjectiveC,yunleihao/LinqToObjectiveC,yunleihao/LinqToObjectiveC,suxinde2009/LinqToObjectiveC,ColinEberhardt/LinqToObjectiveC