title: Python3 result on list appending item appending another item needs more explanation
body: <p>There is a scenario in which I got a little confused.  </p>
  
  <pre class="lang-py prettyprint-override"><code>item = [5]
  item.append(item.append(6))
  print(item)
  
  ## This will result
  [5, 6, None]
  </code></pre>
  
  <p>Why there is None as the last element of the list?</p>