Minor corrections and improvements in Any*Vec docstrings

[tobycheese]

Fixed some small things in docstrings and in an exception over which I stumbled while making myself familiar with gensim:

• Docstrings still mention 'sentences argument, while in code this is now called 'documents'
• Clarify that when not passing training data, the model is left uninitialized
• A generator is also an iterator. 'Sequence' might be a better term (forgotten to mention that one in my commit msg)

[piskvorky]

Related to #2347.

[menshikh-iv]

Thanks @tobycheese, congratz with the first contribution 🥇