DOC: Corrected syntax for appending at certain frame (NetCDF only)

one or more keyword/value pairs may be appended :l

these keywords apply to various dump styles :l

keyword = {append} or {buffer} or {element} or {every} or {fileper} or {first} or {flush} or {format} or {image} or {label} or {nfile} or {pad} or {precision} or {region} or {scale} or {sort} or {thresh} or {unwrap} :l

  {append} arg = {yes} or {no} or {at} N

  {append} arg = {yes} or {no} or {yes at} N

    N = index of frame written upon first dump

  {buffer} arg = {yes} or {no}

  {element} args = E1 E2 ... EN, where N = # of atom types