This fix is based on the comment in https://github.com/docker/docker/pull/27567#discussion_r86910604 Basically, in the help output of `docker service update`, the `--xxx-add` flags typically have plural forms while `--xxx-rm` flags have singular forms. This fix updates the help output for consistency. This fix also updates the related docs in `service_update.md`. The help output in `service_update.md` has been quite out-of-sync with the actual output so this fix replaces the output with the most up-to-date output. This fix is related to #27567. Signed-off-by: Yong Tang <yong.tang.github@outlook.com> Upstream-commit: 1b42d9d0db4f1eadf0a70424ff829bb3346f7bd9 Component: engine