(DOCSP-37743): Add note about upgrading to sharded cluster and adding additional shards (#39) (#43)

* add note about upgrading multi-sharded

* tech review feedback

Author: davidhou17

source/atlasdeployment-custom-resource.txt

@@ -656,6 +656,14 @@ examples, and the |api| documentation.
         - :manual:`Sharding </sharding>`
         - :ref:`server-number-costs`
 
+   .. note:: 
+
+      To ensure consistent data distribution, you can't increase the number 
+      of shards in the same update request when upgrading your {+cluster+} 
+      from a replica set to a sharded {+cluster+}. You can add additional shards 
+      when your {+cluster+} has upgraded and you've ensured all clients have 
+      reconnected to the MongoDB router.
+
 .. setting:: spec.deploymentSpec.replicationSpecs.regionConfigs
 
    *Type*: array