cr-buildbucket: updated docs
cr-buildbucket: updated docs

TBR=sergeyberezin@chromium.org, vadimsh@chromium.org
BUG=

Review URL: https://codereview.chromium.org/821393007

nicko96/Chrome-Infra,nicko96/Chrome-Infra,nicko96/Chrome-Infra,nicko96/Chrome-Infra,nicko96/Chrome-Infra