Corrected description of --sig-proxy

Signal proxy does work only in non-TTY mode (--tty=false). Man pages and
commands should not lie about it.

Signed-off-by: Michal Minar <miminar@redhat.com>
Updated sig-proxy text also in zsh completion script

Signed-off-by: Michal Minar <miminar@redhat.com>
Merge pull request #9144 from miminar/sigproxy_tty

Corrected description of --sig-proxy