Document  side effect on the --userns=host flag (#7537)
Document  side effect on the --userns=host flag (#7537)

* Document  side effect on the --userns=host flag

As explained in https://github.com/moby/moby/issues/34064#issuecomment-430985552

* Fixed typos & added reference to binaries with the setuid flag

londoncalling/docker.github.io,londoncalling/docker.github.io,londoncalling/docker.github.io,docker/docker.github.io,thaJeztah/docker.github.io,londoncalling/docker.github.io,thaJeztah/docker.github.io,thaJeztah/docker.github.io,docker/docker.github.io,thaJeztah/docker.github.io,docker/docker.github.io,thaJeztah/docker.github.io,docker/docker.github.io,docker/docker.github.io,londoncalling/docker.github.io