Update Documentation of BigQuery source/sink to include the exact format of data read/written

[kennknowles]

Currently documentation at following location simply mentions that BigQuery source/sink reads/writes dictionaries. This documentation should be updated to include examples.

https://github.com/apache/beam/blob/master/sdks/python/apache_beam/io/gcp/bigquery.py#L22

Imported from Jira BEAM-3090. Original Jira may contain additional context.
Reported by: chamikara.