Document that sharded clusters cannot be directly downgraded to replica sets in Atlas docs

Hi team,

We have had a number of customers who have deployed sharded clusters and subsequently asked us how to automatically convert back to replica sets. This is not possible and probably should be made clearer in our documentation. Possibly in the Scale your Replica Set to a Sharded Cluster and Deploy a Sharded Cluster sections of the documentation.

Thanks,
Justin

Writers note: I added a note to the suggested documentation and also to the Create and Modify API endpoint documentation.