message: [tf.data] update MapDefun apidef
diff: @@ -41,5 +41,16 @@ END
  }
  summary: <<END
  Maps a function on the list of tensors unpacked from arguments on dimension 0.
  + The function given by `f` is assumed to be stateless, and is executed
  + concurrently on all the slices; up to batch_size (i.e. the size of the 0th
  + dimension of each argument) functions will be scheduled at once.
  +
  + The `max_intra_op_parallelism` attr, which defaults to 1, can be used to
  + limit the intra op parallelism. To limit inter-op parallelism, a user can
  + set a private threadpool on the dataset using `tf.data.Options`'s
  + `ThreadingOptions`.
  +
  + Note that this op is not exposed to users directly, but is invoked in tf.data
  + rewrites.
  END
  }
  