Add service account required permissions to VPC Peering doc page

This page https://docs.couchbase.com/cloud/clouds/private-network.html explains in good detail how to set up VPC peering.  However, it does not provide any information on what permissions are needed for the required service account.  (My understanding is that these are the "DNS Administrator" and "Compute Network Admin" roles on GPC; probably something similar on AWS.)  Because the whole thing feels if the account does not have correct permissions, we should make them explicit in the doc page.