Fix docstrings for gensim.utils (#1797)

* Add docstrings in numpy-style fromat

* fix PEP8

* remove outdated "hack" (smart_open is core dependency right now)

* fix docstrings[1]

* remove unused internal class

* fix docstrings[2]

* fix docstrings[3]

* fix docstrings[4]

* fix docstrings[5]

* fix docstrings[6]

* fix docstrings[7]

* fix docstrings[8]

* add missing `pattern` to doc dependencies

* fix docstrings[9]

* fix docstrings[10]