There's a few things that keep confusing me here. As far as I understand, `_aggregateMembersDocuments` is used to index the members of a resource (group or content), so that the members can be searched. It looks like `_aggregateMembershipsDocuments` is used to index the group memberships of a principal, so that we can search through a user's membership. However, the documentation isn't confirming this.

One thing that isn't helping is that we use `resourceId` for what I think is a principal, whilst we clearly differentiate between resources and principals in the authz api.