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)
add Docker version and branch to html header

Docker-DCO-1.1-Signed-off-by: Sven Dowideit <SvenDowideit@docker.com> (github: SvenDowideit)
Remove extra "Docker" from docs index
Merge pull request #6761 from fgrehm/patch-1

Remove extra "Docker" from docs index
Merge pull request #6631 from SvenDowideit/add-versioning-to-docs-output

Add versioning to docs output
Merge pull request #6544 from mheon/sigproxy_docs

Update --sig-proxy documentation