Port redirection doc

This PR clarifies the documentation surrounding port redirection.

I had mentionned being confused in #2415 but the following two links cleared everything for me:
+ http://docker.readthedocs.org/en/latest/commandline/cli/#run
+ http://docs.docker.io/en/latest/examples/linking_into_redis/

I put the information from these two links back in the port direction use page.