Add exit status to docker exec manpage
Add exit status to docker exec manpage

There's little way of knowing what each exit status means at present
because it's not documented. I'm assuming they are the same as docker
run.

Signed-off-by: Eric Curtin <13242e7d2f7e10c251cda8d8b326a86e0569de1d@gmail.com>

cpuguy83/cli,thaJeztah/cli,thaJeztah/cli,cpuguy83/cli,docker/cli,docker/cli