added comment to build_index_mappings

def _build_index_mappings(name, data_prefix, documents, sizes,

                          num_samples, seq_length, seed):

    """doc-idx, sample-idx, and shuffle-idx."""

    """Build doc-idx, sample-idx, and shuffle-idx.

    doc-idx: is an array (ordered) of documents to be used in training.

    sample-idx: is the start document index and document offset for each

       training sample.

    shuffle-idx: maps the sample index into a random index into sample-idx.

    """

    # Number of tokens in each epoch and number of required epochs.

    tokens_per_epoch = _num_tokens(documents, sizes)

    num_epochs = _num_epochs(tokens_per_epoch, seq_length, num_samples)