Please add documentation on using multiple fetch repositories

When using multiple git repositories during the fetch stage, it's unclear how the "fetched" repositories are stored temporarily by kapp-controller.

dkalinin was super helpful in pointing out that the git repos are put into numbered repos. Be also helpful to document a more clear example as the Helm example using stable/nginx doesn't mention which Helm repo it is pulling from.

[Slack discussion](https://kubernetes.slack.com/archives/CH8KCCKA5/p1636986684394000)