bigquery: improve package doc
bigquery: improve package doc

Change-Id: Ibfa4de8c59f1b6a548f8d078ec8d3bd3cc002b23
Reviewed-on: https://code-review.googlesource.com/8574
Reviewed-by: Sarah Adams <2b342d1c32491b4e7587e4a0e291a5f0f95d4167@google.com>
Reviewed-by: Michael McGreevy <e70fec140fa6773c6cfd45cb9b436bd6334f4c24@golang.org>

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