[logdog] Improve Makefile a bit.
[logdog] Improve Makefile a bit.

Add missing documentation and a "rollback" target for Kubernetes
clusters.

TBR=iannucci@chromium.org
BUG=chromium:712403
TEST=None

Change-Id: I8bf285a07097f5f80eca686cd07567366e984c6e
Reviewed-on: https://chromium-review.googlesource.com/700405
Reviewed-by: Daniel Jacques <8400006a9338a0893dca0101ad49838955bd6b2c@chromium.org>
Commit-Queue: Daniel Jacques <8400006a9338a0893dca0101ad49838955bd6b2c@chromium.org>

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