[docs] define clusterRole for appset-controller (watching many namespaces)

[TheRealNoob]

Checklist:

• I've searched in the docs and FAQ for my answer: https://bit.ly/argocd-faq.

Describe the bug

This section of the app-in-any-namespace documentation details the additional RBAC manifests that must manually be made when configuring argo to deploy apps from many namespaces. These are super helpful and I was very appreciative to have them.

When I went to configure the same behavior for appplicationSets I found that there wasn't equivilant docs on the permissions required for that serviceAccount. From my small tinkering so far (before giving up and doing allow all on argoproj.io resources) I found it that it requires the below permissions. It would be really helpful to get a concrete list of the required permissions from the people more informed than I. I'd be happy to open all the relevant PRs if so. Thank you!

rules:
  - apiGroups:
      - "argoproj.io"
    resources:
      - "applications"
      - "applicationsets"
    verbs:
      - create
      - get
      - list
  - apiGroups:
      - ""
    resources:
      - secrets
      - configmaps
    verbs:
      - get
      - list
      - watch