Updated the documentation to to clarify random port mapping when using -...

The documentation for `docker run` does not clearly explain that using -P will map exposed ports to random high ports 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 can create confusion for users (see http://stackoverflow.com/q/25920723/1027148 as an example).