title: How to specify different return types in python docstring
body: <p>I'm currently writing documentation for my python package using Sphinx and the autodoc plugin. 
  For a function return value I can e.g. write <code>:returns: int: count</code> which tells sphinx that there is a return value of type int, named count.</p>
  
  <p>I now got a function which gets me predecessors of items in my database:</p>
  
  <pre><code>def get_previous_release(release_id):
      """ Holt Vorgängeritem eines Items mit der ID release_id
  
          :param release_id: ID des items für das Release
          :type release_id: int
  
      """
  
      if not isinstance(release_id, int):
          raise ValueError('get_previous_release expects an Integer value for the parameter release_id')
  
      try:
          release = my_queries.core.get_by_id(release_id)
      except IndexError:
          raise LookupError('The item with id {} could no be found'.format(release_id))
  
      if 'Alpha-Release' in release.name:
          release = AlphaRelease(release.key, release.name, release.state)
      elif 'Beta-Release' in release.name:
          release = BetaRelease(release.key, release.name, release.state)
      elif '-Release' in release.name:
          release = VRelease(release.key, release.name, release.state)
      else:
          raise TypeError('The item with the id {} does not contain \'-Release\' in the Summary ' + \
                          'and is therefore not considered a Release')
  
      previous_release = release.get_predecessor()
  
      if not previous_release:
          raise LookupError('Could not find a predecessor for item with id {}'.format(release_id))
  
      return previous_release
  </code></pre>
  
  <p>As you can see, it fetches the original item and either returns an instance of class <code>AlphaRelease</code>, <code>BetaRelease</code> or <code>VRelease</code> depending on the content of the field <code>name</code> of the items.</p>
  
  <p>What is best practice to define a return value with various possible types in the docstring?</p>