docs: Provider can deploy to pre-existing namespace

[mloskot]

If flux-system already exists, then the provider will
use it as scope for installation of Flux manifests.

It seems important to document this behaviour to
indicate there is no need for to add a boolean
e.g. create_namespace=true|false.

---

I have just tested this behaviour myself using flux_bootstrap_git against pre-existing flux-system namespace with pre-existing Secret with Git SSH credentials:

resource "flux_bootstrap_git" "gitops" {
  path      = "clusters/test"
  namespace = "flux-system"

  disable_secret_creation = true
  secret_name             = local.flux_github_secret
}

[stefanprodan]

LGTM

Thanks @mloskot