class_docstr: object for annotation documents
  
  Args:
      self.doc_text (str): document text
      self.annotation_set (list(str)): list of all annotations in doc
      self.spans (list(dict)): list of span dict, each span_dict is structured as follows:
          { 'end': (int), 'start': (int), 'type': (str) string of annotation }
code: def __init__(self):
      self.doc_text = None
      self.annotation_set = []
      self.spans = []
  
imports: from __future__ import unicode_literals, print_function, division, absolute_import
  import json
label_desc: Use image node_id 1 to create a new HighLevelDoc object  with example: obj = HighLevelDoc()