Make documentation relevant

[DifferentialOrange]

This PR updates existing tarantool-python module documentation and adds descriptions where they were missing. See commit messages for more detailed info.

To build documentation, first you must install its build requirements:

pip install -r requirements-doc.txt

Then run

make docs

You may host local documentation server with

python -m http.server --directory build/sphinx/html

Open localhost:8000 in your browser to read the docs.

Closes #67

This PR does not yet provide publishing documentation with CI, see https://github.com/tarantool/tarantool-python/issues/238. readthedocs module documentation will remain outdated until this PR would be reviewed thoroughly.

[DifferentialOrange]

Add CHANGELOG entries where it seemed relevant.