bigquery: update package doc
bigquery: update package doc

Change-Id: Iaff965530f01e3443046bc5cf30ab2368847036c
Reviewed-on: https://code-review.googlesource.com/9836
Reviewed-by: Jaana Burcu Dogan <347c19a639ce752a783b4ea325d6539ab4b4a262@google.com>

michael-grosshans/google-cloud-go,michael-grosshans/google-cloud-go