Can't resize server pools (#1246)

Explicitly note in the concepts page that you can't change the size of a
server pool after it's created.

Not staged

source/operations/concepts.rst

@@ -107,6 +107,7 @@ MinIO :ref:`distributed deployments <minio-mnmd>` support expansion and decommis
 
 Expansion consists of adding one or more :ref:`server pools <minio-intro-server-pool>` to an existing deployment.
 Each server pool consists of dedicated nodes and storage that contribute to the overall capacity of the deployment.
+Once you create a server pool you cannot change its size, but you can add or remove capacity at any time by adding or decommissioning pools.
 
 .. cond:: linux