DOCSP-47926 -- add unidirectional scaling note (#2076)

* DOCSP-47926 -- add unidirectional scaling note

* DOCSP-47926 -- unidirectional scaling external review

* DOCSP-47926 -- external review

Author: jvincent-mongodb

source/multi-cluster-sharded-cluster.txt

@@ -107,6 +107,19 @@ But in the overrides we change shard ``sc-0`` to have five members, two on clust
 ``1``, two on cluster ``2``, and cluster ``3`` still has one shard as per the 
 default. 
 
+.. note:: Only unidirectional scaling is supported.
+
+   When you scale shard, config server, or mongos replicas you can only scale 
+   resources up or down in order to ensure the correctness of scaling. This rule 
+   applies to both single |k8s| cluster deployments and those that are 
+   distributed across multiple |k8s| clusters. Additionally, in multi-cluster 
+   deployments the unidirectional scaling rule applies to all resource types.
+   For example, you can't add more nodes (scaling up) to shards while simultaneously 
+   removing (scaling down) config servers or mongos. You can no longer "move" one 
+   node from one cluster to another without changing the total number of members. 
+   Rather, you must first perform a scale-up operation and then execute a separate 
+   change for scaling down. 
+
 This example configuration also shifts (with ``shardOverrides``) the primary to cluster ``2``, 
 for the shard ``sc-0``, which can reduce latency for users operating in the region where  cluster ``2`` 
 is located. In this way, you still have resilience across the clusters 
@@ -143,6 +156,7 @@ defined in the override definition are removed.The ``merge`` value indicates
 that a single field is updated, and the ``replace`` value indicates that the 
 complete parent object is overridden.
 
+
 Customize Persistence and Statefulset Settings
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~