message: Improve API doc for ZipDataset
diff: op {
  graph_op_name: "ZipDataset"
  visibility: HIDDEN
  + in_arg {
  + name: "input_datasets"
  + description: <<END
  +List of `N` variant Tensors representing datasets to be zipped together.
  +END
  + }
  + attr {
  + name: "output_types"
  + }
  + attr {
  + name: "output_shapes"
  + }
  + attr {
  + name: "N"
  + description: "The length of `input_datasets`"
  + }
  summary: "Creates a dataset that zips together `input_datasets`."
  + description: <<END
  +The resulting dataset will produce a stream of elements, where elements `k`
  +contains the concatenation of the `k`th elements from each of the input
  +datasets.
  +
  +The stream will stop as soon as any one of the input datasets runs out
  +of elements, and noerror will be raised if the other datasets contain
  +additional data.
  +
  +`ZipDataset` invokes its input datasets serially, one record at a time.
  +
  +Most instances of this type of dataset come from the `zip()` method in
  +`tf.data.DataSet`. The `hoist_random_uniform`
  +static optimization also creates instances of `ZipDataset` while rewriting
  +stateful functions into stateless functions.
  +END
  }
  