Make docstring follow PEP 257

[Linchin]

Coming from this comment.

A lot of the docstrings in this repo are not properly following PEP 257, leading to issue rendering in the documentation. For example, this class property has a redundant type annotation, and it is not recognized by the doc:

We can audit the repo and update the docstrings to be PEP 257 compliant.

[parthea]

Transferring to google-cloud-python as the code for python-bigquery will be migrated there in 1-3 weeks.

[parthea]

Transferring this issue back to python-bigquery.