bigtable: clean up documentation for cbt
bigtable: clean up documentation for cbt

Change-Id: Iad5c1ff153f6496ca7ce4e038f077936899073dc
Reviewed-on: https://code-review.googlesource.com/c/gocloud/+/45470
Reviewed-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Tyler Bui-Palsulich <b41e4aa57d2ad7eb1b29835a1b483e209dcc3c0e@google.com>

googleapis/cloud-bigtable-cbt-cli,googleapis/cloud-bigtable-cbt-cli