Fixes #9305: Document agent-server compatibility limits for 4.0

12_upgrade/00_upgrade.txt

@@ -14,8 +14,18 @@ occur during the upgrade process.
 
 Migration from 3.1 or 3.2 are supported, so you can upgrade directly to 4.0.
 
-If you migrate from a 3.1 release older than 3.1.11, you have to upgrade
-you server before your agents (as 4.0 agents are not compatible with old servers).
+[WARNING]
+
+====
+
+As we changed the communication protocol between agent end server in 4.0,
+some networking issues may appear when using 4.0 agents with older servers with the
+reverse DNS lookup option disabled in the settings (*Security* ->
+*Use reverse DNS lookups on nodes to reinforce authentication to policy server*).
+
+Therefore, you need to upgrade your server to 4.0 *before* upgrading the nodes.
+
+====
 
 ==== Upgrade from Rudder 3.0 or older