Clarify registry image names in documentation

In https://docs.gitlab.com/ce/user/project/container_registry.html#build-and-push-images it sounds like we’re encouraging people to use 3 level image names by putting /image in the first example. Can we assume that the most common use will be a single image per project, and talk about the optional image name separately? The help text on the registry page doesn’t help, but there’s a separate issue to address that.