title: Understanding the role of the function build_vocab in Doc2Vec
body: <p>I have recently started studying Doc2Vec model.
  I have understood its mechanism and how it works.
  I'm trying to implement it using gensim framework.
  I have transormed my training data into TaggedDocument.
  But i have one question :
  What is the role of this line <code>model_dbow.build_vocab([x for x in tqdm(train_tagged.values)])</code> ?
  is it to create random vectors that represent text ?
  Thank you for your help</p>