add note on download_localhost (#7623)

It can be counterintuitive for the downloaded files to show up on local host when download_localhost is false, this explains the reason.

docs/downloads.md

@@ -9,7 +9,7 @@ Kubespray supports several download/upload modes. The default is:
 There is also a "pull once, push many" mode as well:
 
 * Setting ``download_run_once: True`` will make kubespray download container images and binaries only once and then push them to the cluster nodes. The default download delegate node is the first `kube_control_plane`.
-* Set ``download_localhost: True`` to make localhost the download delegate. This can be useful if cluster nodes cannot access external addresses. To use this requires that docker is installed and running on the ansible master and that the current user is either in the docker group or can do passwordless sudo, to be able to access docker.
+* Set ``download_localhost: True`` to make localhost the download delegate. This can be useful if cluster nodes cannot access external addresses. To use this requires that the container runtime is installed and running on the Ansible master and that the current user is either in the docker group or can do passwordless sudo, to be able to use the container runtime. Note: even if `download_localhost` is false, files will still be copied to the Ansible server (local host) from the delegated download node, and then distributed from the Ansible server to all cluster nodes.
 
 NOTE: When `download_run_once` is true and `download_localhost` is false, all downloads will be done on the delegate node, including downloads for container images that are not required on that node. As a consequence, the storage required on that node will probably be more than if download_run_once was false, because all images will be loaded into the docker instance on that node, instead of just the images required for that node.