[FAB-13253] Improve SDK compatibility doc

Clarify support statement.

Change-Id: I248b96a417473b27bd1ed30cb8face2f949401c2
Signed-off-by: David Enyeart <enyeart@us.ibm.com>

docs/source/upgrading_your_network_tutorial.rst

@@ -393,7 +393,12 @@ To learn how to upgrade your Fabric CA server, click over to the
 Upgrade Node SDK clients
 ~~~~~~~~~~~~~~~~~~~~~~~~
 
-.. note:: Upgrade Fabric CA before upgrading Node SDK clients.
+.. note:: Upgrade Fabric and Fabric CA before upgrading Node SDK clients.
+          Fabric and Fabric CA are tested for backwards compatibility with
+          older SDK clients. While newer SDK clients often work with older
+          Fabric and Fabric CA releases, they may expose features that
+          are not yet available in the older Fabric and Fabric CA releases,
+          and are not tested for full compatibility.
 
 Use NPM to upgrade any ``Node.js`` client by executing these commands in the
 root directory of your application: