This fix updates docs of `label` filter for `docker system prune`.
This fix is related to #30740 and #29999, and specifically to comment
https://github.com/docker/docker/pull/30740#issuecomment-293012957.
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>
Upstream-commit: 40bb4a3b0e
Component: cli