[4230] Updated User's Guide per review comment.

Explained that the "duid" parameter is accepted for DHCPv4
reservations but shouldn't be used because will be
misinterpreted.

doc/guide/dhcp4-srv.xml

@@ -2412,7 +2412,11 @@ It is merely echoed by the server
     <note><para>In Kea 1.0.0 it is only possible to create host reservations
     using client's hardware address. Host reservations by client identifier
     (or DUID) are not supported in this version of Kea. This capability will
-    be implemented in Kea 1.1.0.</para></note>
+    be implemented in Kea 1.1.0. Currently, the configuration parsing code
+    will accept the "duid" parameter in the reservation configuration, but
+    the server will misinterpret its value. Therefore, this parameter MUST
+    NOT be used until the client identifier based host reservations are
+    properly implemented and documented.</para></note>
 
     <para>The following example shows how to reserve addresses for specific
     hosts: