Update the documentation to to clarify random port mapping when using -P on docker run

The documentation for `docker run` does not clearly explain that using `-P` will map exposed ports to a random high port on the host.

Although this is documented in <http://docs.docker.com/userguide/dockerlinks/#network-port-mapping-refresher>, it should also be clearly specified in the `docker run` documentation as it's confusing for users (see  <http://stackoverflow.com/q/25920723/1027148> as an example).