Document accessing the vocabulary of a *2vec model

[mpenkov]

@andreamoro like this:

from gensim.test.utils import common_texts
from gensim.models import Word2Vec

model = Word2Vec(common_texts, size=100, window=5, min_count=1, workers=4)
word_vectors = model.wv
print(list(word_vectors.vocab.keys()))

['minors', 'graph', 'system', 'trees', 'eps', 'computer', 'survey', 'user', 'human', 'time', 'interface', 'response']

Documentation of KeyedVectors = the class holding the trained word vectors.

@mpenkov listing the model vocab is a reasonable task, but I couldn't find it in our documentation either. I had to look at the source code. Maybe we can add it somewhere? API ref? Tutorial? @andreamoro where would you expect / look for this information?

Originally posted by @piskvorky in #2422 (comment)