[doc] update bqschemaupdater docs
[doc] update bqschemaupdater docs

Bug:798427
Change-Id: Ie0e4f0b606f2a42a2f80880f4fd4573f038f9cc4
Reviewed-on: https://chromium-review.googlesource.com/850947
Reviewed-by: Nodir Turakulov <ef4933a197ef7b4b3f55f1bec4942aead3637a2a@chromium.org>
Commit-Queue: Katie Thomas <041daa1eaa3c22c04a99fd8b5f72449daed60d55@google.com>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go