[Atlas] GCP Private Service Connect confusion in Docs

In our documentation one of our customers was confused by the considerations for the GCP PSC: https://docs.atlas.mongodb.com/security-private-endpoint/#regionalized-private-endpoints-for-multi-region-sharded-clusters

They thought it would be mandatory that a GCP PSC could only be set up for multi-region sharded cluster. I think adding an "optional" to the headline of the subsection could reduce the number of confused customers at this point.