Sphinx warnings

[tmylk]

Sphinx 1.5.3 raises 108 warnings when generating gensim docs when running

cd docs/src
make clean html 

All the files are generated correctly even though Sphinx labels some of the warnings as errors.

The easiest warnings to fix are in the rst files of type Title underline too short.

The warnings in the py file are hard to fix because the line numbers listed in the error messages don't correspond to the line numbers in the files. For example, there is no docstring on line 41 though sphinx reports
coherencemodel.py:docstring of gensim.models.coherencemodel.CoherenceModel:41: ERROR: Unexpected indentation.

[menshikh-iv]

Finished in #1442