clarify --runtime-flag of run command
clarify --runtime-flag of run command

Signed-off-by: Tomas Tomecek <ttomecek@redhat.com>

Fixes #178

Closes: #182
Approved by: rhatdan

rhatdan/buildah,rhatdan/buildah,TomSweeneyRedHat/buildah,ipbabble/buildah,nalind/buildah-1,projectatomic/buildah,TomSweeneyRedHat/buildah,rhatdan/buildah,nalind/buildah-1,projectatomic/buildah,nalind/buildah-1,projectatomic/buildah,nalind/buildah,ipbabble/buildah,nalind/buildah,TomSweeneyRedHat/buildah