Document the limitation of server group data placement with respect to replica count in Couchbase Server 7.1

As described in MB-50712, in 7.1 if there are at least two server groups the number of replicas placed for a bucket will be at most one less than the number of server groups. An example will probably help. E.g.

• Say there are 2 server groups with 3 KV nodes each
• A bucket is created with 2 replicas

In 7.1 we will only place 1 replica for the bucket and we will not place the replica in a server group where the active vbucket is placed. (This is the work that is done for MB-43012.)

We should update the Server Group page in the docs (https://docs.couchbase.com/server/current/learn/clusters-and-availability/groups.html#server-groups-and-vbuckets) to describe this limitation. We are working on addressing the limitation.