text: Title: Word2Vec docs indicate sents can be a generator, but it can't
  Body: <!--
  **IMPORTANT**:
  
  - Use the [Gensim mailing list](https://groups.google.com/forum/#!forum/gensim) to ask general or usage questions. Github issues are only for bug reports.
  - Check [Recipes&FAQ](https://github.com/RaRe-Technologies/gensim/wiki/Recipes-&-FAQ) first for common answers.
  
  Github bug reports that do not include relevant information and context will be closed without an answer. Thanks!
  -->
  
  #### Problem description
  
  Tried to pass a generator as the `sentences` argument when training Word2Vec, as suggested [in the docs](https://radimrehurek.com/gensim/models/word2vec.html#usage-examples:~:text=sentences%20can%20be%20a%20generator). But then I get this error:
  
      TypeError: You can't pass a generator as the sentences argument. Try a sequence.
  
  #### Steps/code/corpus to reproduce
  
  ```
  
  from gensim.models import Word2Vec
  
  def get_sents():
      for ii in range(100):
          yield ['hi']
  
  sents = get_sents()
  model = Word2Vec(sents, size=100, window=5, min_count=1, workers=4)
  ```
  
  #### Versions
  
  Please provide the output of:
  
  ```python
  import platform; print(platform.platform())
  import sys; print("Python", sys.version)
  import struct; print("Bits", 8 * struct.calcsize("P"))
  import numpy; print("NumPy", numpy.__version__)
  import scipy; print("SciPy", scipy.__version__)
  import gensim; print("gensim", gensim.__version__)
  from gensim.models import word2vec;print("FAST_VERSION", word2vec.FAST_VERSION)
  ```
  ```
  Linux-5.8.5-arch1-1-x86_64-with-glibc2.2.5
  Python 3.8.5 (default, Jul 27 2020, 08:42:51) 
  [GCC 10.1.0]
  Bits 64
  NumPy 1.19.1
  SciPy 1.4.1
  gensim 3.8.3
  FAST_VERSION 1
  ```
  