updating description for repo create --proxy-host option

https://bugzilla.redhat.com/show_bug.cgi?id=1086855

Docs already seem to mention the right way - http://pulp-rpm-user-guide.readthedocs.org/en/latest/recipes.html#use-a-proxy

I have just updated the description for --proxy-host to say url instead of hostname. I also investigated changing --proxy-host option to --proxy-url. Just changing it on cli is not invasive, but that will be different from global proxy config which is --proxy-host. If we want to change both, it is quite a bit invasive which which will require changing many files, unit tests and a migration to update previously created importer configs. I don't think it makes sense to do this for 2.4.