Add explanation why set masks after successful OPC-UA conn

RCUSCC/RCUSCC/RCUSCC.py

@@ -367,7 +367,13 @@ class RCUSCC(Device):
         # Start keep-alive
         self.opcua_connection.start()
 
-        # Set the masks
+        # Set the masks.
+        #
+        # Attention!
+        # Set the masks only after the OPCUA connection has been
+        # established!  The setting of the masks needs to go through
+        # to the server.
+        #
         # TODO
         # Read default masks from config DB
         self.write_RCU_mask_RW(self._RCU_mask_RW)