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)

docs/sources/reference/commandline/cli.md

@@ -135,7 +135,7 @@ like this:
     Attach to a running container
 
       --no-stdin=false    Do not attach stdin
-      --sig-proxy=true    Proxify all received signal to the process (even in non-tty mode)
+      --sig-proxy=true    Proxify received signals to the process (even in non-tty mode). SIGCHLD is not proxied.
 
 The `attach` command will allow you to view or
 interact with any running container, detached (`-d`)
@@ -898,7 +898,7 @@ removed before the image is removed.
       -P, --publish-all=false    Publish all exposed ports to the host interfaces
       --privileged=false         Give extended privileges to this container
       --rm=false                 Automatically remove the container when it exits (incompatible with -d)
-      --sig-proxy=true           Proxify all received signal to the process (even in non-tty mode)
+      --sig-proxy=true           Proxify received signals to the process (even in non-tty mode). SIGCHLD is not proxied.
       -t, --tty=false            Allocate a pseudo-tty
       -u, --user=""              Username or UID
       -v, --volume=[]            Bind mount a volume (e.g. from the host: -v /host:/container, from docker: -v /container)