Is your feature request related to a problem? Please describe.
There is no mention in the current docs if a google.cloud.bigquery.Client() instance can safely be shared among threads. This question is similar to googleapis/google-cloud-python#7421

Describe the solution you'd like
Some docs at e.g. https://googleapis.dev/python/bigquery/latest/generated/google.cloud.bigquery.client.Client.html#google.cloud.bigquery.client.Client explaining if a google.cloud.bigquery.Client() can safely be reused among threads.

Describe alternatives you've considered
I searched this repository for mentions of "bigquery" and "thread safe" but could not find any specific information.