Add a bit more documentation flavor to cmpbin.
Add a bit more documentation flavor to cmpbin.

TBR=dnj@chromium.org, estaab@chromium.org, tandrii@chromium.org, vadimsh@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1322713002

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