Make docstring follow PEP 257

Coming from [this comment](https://github.com/googleapis/python-bigquery/pull/2333#discussion_r2560873839).

A lot of the docstrings in this repo are not properly following [PEP 257](https://peps.python.org/pep-0257/#multi-line-docstrings), leading to issue rendering in the documentation. For example, this [class property](https://github.com/googleapis/python-bigquery/blob/0529726400b1df4ade75cc3c9a829632ccb72eb8/google/cloud/bigquery/schema.py#L285) has a redundant type annotation, and it is not recognized by the [doc](https://docs.cloud.google.com/python/docs/reference/bigquery/latest/google.cloud.bigquery.schema.SchemaField#google_cloud_bigquery_schema_SchemaField_field_type):

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