Explicitly mention that '-P' maps to random ports

as noted in https://github.com/boot2docker/boot2docker/issues/690

Signed-off-by: Sven Dowideit <SvenDowideit@docker.com>

Docker-DCO-1.1-Signed-off-by: Sven Dowideit <SvenDowideit@docker.com> (github: SvenDowideit)