update rolling doc string
update rolling doc string

pydata/xarray,jhamman/xarray,rabernat/xray,jhamman/xarray,chunweiyuan/xarray,markelg/xray,pydata/xarray,shoyer/xray,jcmgray/xarray,rabernat/xray,shoyer/xarray,xray/xray,markelg/xray,markelg/xray,jhamman/xarray,pydata/xarray,shoyer/xarray,jhamman/xray,NicWayand/xray