SERVER-5733 chunkSize documentation regarding changing chunk size

Author: Sam Kleinman

source/reference/configuration-options.txt

@@ -709,4 +709,9 @@ Sharding Cluster Options
    unnecessary migrations. However, in some circumstances it may be
    neccessary to set a different chunk size.
 
-   This setting only affects :program:`mongos` processes.
+   This setting only affects :program:`mongos` processes. Furthermore,
+   :setting:`chunkSize` *only* sets the chunk size when initializing
+   the cluster for the first time. If you modify the run-time option
+   later, the new value will have no effect. See the
+   ":ref:`sharding-balancing-modify-chunk-size`" procedure if you
+   need to change the chunk size on an existing shard cluster.

source/reference/mongos.txt

@@ -200,6 +200,12 @@ Options
    some circumstances it may be neccessary to set a different chunk
    size.
 
+   This option *only* sets the chunk size when initializing the
+   cluster for the first time. If you modify the run-time option
+   later, the new value will have no effect. See the
+   ":ref:`sharding-balancing-modify-chunk-size`" procedure if you
+   need to change the chunk size on an existing shard cluster.
+
 .. option:: --ipv6
 
    Enables IPv6 support to allow clients to connect to :program:`mongos`