BigQuery: Convert Sphinx-style docstrings to Google style

[plamut]

Following the comment thread on another PR, it turned out that job.py, and possibly other modules as well, still use the Sphinx-style docstrings.

These docstrings should be converted to Google-style docstrings as the Python coding style guideline mandates.

Examples of Google-style docstrings:
https://sphinxcontrib-napoleon.readthedocs.io/en/latest/example_google.html#example-google

[Patil2099]

@plamut @IlyaFaer Can I work on this?

[IlyaFaer]

@Patil2099, I've already wrote raw script for massive docs updating. Pushed internally just a few minutes ago - was gonna show to @tswast and ask about how we should deal with this changes, and maybe there is something to do "by the way"

[Patil2099]

@IlyaFaer Maybe I can help in that.

[plamut]

@IlyaFaer I've already done quite some work on this in BigQuery, but if we are going to use a mass update script, I'll put that aside. Thanks for mentioning the script here!

@Patil2099 I am fine with you working on this, just please coordinate with @IlyaFaer to not unnecessarily repeat any work.