function _setArrowPlacement(sPlacement){
        htVar.htArrowPlacement = htVar.htArrowPlacement || {
            "top": "bottom",
            "bottom": "top"
        };

        htElement.welCommentWrap.removeClass("arrow-top arrow-bottom")
            .addClass("arrow-" + htVar.htArrowPlacement[sPlacement]);
    }
Set arrow placement of welCommentWrap
arrow will be placed on opposite side to sPlacement

@param sPlacement Where to show commentBox from welTarget (top or bottom)
@private