message: Reword doc for num_parallel_calls to be less implementation-specific
diff: @@ -32,14 +32,9 @@ END
  in_arg {
  name: "num_parallel_calls"
  description: <<END
  -If this input is >=1, it bounds the number of datasets from which this instance
  -of `ParallelInterleaveDatasetV2` will fetch simultaneously.
  -The actual number of parallel calls will depend on the `cycle_length`
  -parameter, the number of available CPUs, and the number of available datasets.
  -
  -If this input is set to -1 (i.e. the constant `tf.data.experimental.AUTOTUNE`
  -in the Python API), then the dataset's kernel will choose an upper bound
  -automatically.
  +Determines the number of threads that should be used for fetching data from
  +input datasets in parallel. The Python API `tf.data.experimental.AUTOTUNE`
  +constant can be used to indicate that the level of parallelism should be autotuned.
  END
  }
  attr {
  