Improve doc gsubtreed bootstrap_from_existing doc.
Improve doc gsubtreed bootstrap_from_existing doc.

BUG=467487
R=iannucci@chromium.org

Review URL: https://codereview.chromium.org/1040593003

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