Add mention of fetch module to the copy module docs.

I've run into this twice, and someone today on IRC also mentioned it; the `copy` module makes no mention of the `fetch` module (which is basically `copy` in reverse).

The `fetch` docs *do* mention `copy`. So to help people who are searching for things like "ansible copy files to host" or something like that, it would be good if the copy docs mentioned `fetch` as well.