Improve docs around `upload_policy=skip` behaviour

It is assumed to be the default and expected behavior, not hitting the servers when not necessary, this is why it is not documented that explicitly. But we can definitely improve the docs in to clarify this https://docs.conan.io/2/reference/conanfile/attributes.html#upload-policy

_Originally posted by @memsharded in https://github.com/conan-io/conan/pull/15336#discussion_r1436366611_