Instead of referring to groups, which is a term of art for a different entity,
update the doc comments to more accurately describe what tags are in reference
to the policy document.
Updates #cleanup
Change-Id: Iefff6f84981985f834bae7c6a6c34044f53f2ea2
Signed-off-by: M. J. Fromberger <fromberger@tailscale.com>