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: 5b0ab45a1ef2ce52e78851f49a8390263e642263 Component: engine