add more commenting to smilestokenizer class

class SmilesTokenizer(BertTokenizer):

  """

    Creates the SmilesTokenizer class. The tokenizer heavily inherits from the BERT

    WordPieceTokenizer implementation found in Huggingface's transformers library.

    Creates the SmilesTokenizer class. The tokenizer heavily inherits from the BertTokenizer

    implementation found in Huggingface's transformers library. It runs a WordPiece tokenization

    algorithm over SMILES strings using the tokenisation SMILES regex developed by Schwaller et. al.

    Please see https://github.com/huggingface/transformers

    and https://github.com/rxn4chemistry/rxnfp for more details.