Update documentation noting that SIGCHLD is not proxied.
Update documentation noting that SIGCHLD is not proxied.

Docker's --sig-proxy option sends all signals but one to a container. The
exception s SIGCHLD, which is deliberately ignored as it doesn't make sense to
send such a signal to a process in a container. Documentation updates will make
this less confusing if anyone does attempt to do this.

Docker-DCO-1.1-Signed-off-by: Matt Heon <mheon@redhat.com> (github: mheon)

dnephin/cli,e11137/cli,riyazdf/cli,ashfall/cli,docker/cli,cpuguy83/cli,allencloud/cli,Zebrilee/cli,ashfall/cli,redpanda/cli,thaJeztah/cli,thaJeztah/cli,redpanda/cli,riyazdf/cli,docker/cli,cpuguy83/cli,Zebrilee/cli,e11137/cli,allencloud/cli,dnephin/cli