title: how to use build_vocab in gensim?
body: <ol>
  <li>Build_vocab extend my old vocabulary? </li>
  </ol>
  
  <p>For example, my idea is when I use doc2vec(s) to train a model, it just builds the vocabulary from the datasets.  If I want to extend it, I need to use build_vocab()</p>
  
  <ol start="2">
  <li>Where should I use it?  Should I put it after "gensim.doc2vec()"?  </li>
  </ol>
  
  <p>For example:</p>
  
  <pre><code>sentences = gensim.models.doc2vec.TaggedLineDocument(f_path)
  dm_model = gensim.models.doc2vec.Doc2Vec(sentences, dm=1, size=300, window=8, min_count=5, workers=4)
  dm_model.build_vocab()
  </code></pre>