[3688] Added description of the hostname reservations in the kea guide.

doc/guide/dhcp4-srv.xml

@@ -2124,10 +2124,22 @@ temporarily override a list of interface names and listen on all interfaces.
 
     <section id="reservation4-hostname">
       <title>Reserving a hostname</title>
-      <!-- @todo: replace this with the actual text once #3688 is implemented -->
-      <para>Reserving a hostname is currently not supported. It is possible
-      to specify that information in the configuration file, but that data
-      is not used by the server engine yet.</para>
+      <para>When the reservation for the client includes the <command>hostname
+      </command>, the server will assign this hostname to the client and send
+      it back in the Client FQDN or Hostname option, depending on which of them
+      the client has sent to the server. The reserved hostname always takes
+      precedence over the hostname supplied by the client or the autogenerated
+      (from the IPv4 address) hostname. If the reservation for the hostname is
+      specified the server ignores the <command>qualifying-suffix</command>
+      parameter and treats the value specified in the reservation as a fully
+      qualified name.</para>
+
+      <note>
+        <para>Future Kea versions may provide additional configuration
+        parameters to specify whether the reserved hostname is a fully qualified
+        name or the suffix should be appended.
+        </para>
+      </note>
     </section>
 
     <section id="reservation4-options">