[3872] comment out "DHCP Performance Guide" add dhcpd.8 to See Also

Performance Guide is gone. See ticket #4077.
Also this tool is not kea specific, so reference legacy man page too.

src/bin/perfdhcp/perfdhcp.xml

@@ -853,10 +853,15 @@
     <refsect1>
         <title>SEE ALSO</title>
         <para>
+            <citerefentry><refentrytitle>dhcpd</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
             <citerefentry><refentrytitle>kea-dhcp4</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
             <citerefentry><refentrytitle>kea-dhcp6</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
-            <citetitle>Kea Administrator Guide</citetitle>,
-            <citetitle>DHCP Performance Guide</citetitle>.
+            <citetitle>Kea Administrator Guide</citetitle>.
+<!--
+see http://kea.isc.org/ticket/4077
+            <citetitle>DHCP Performance Guide</citetitle>
+-->
+
         </para>
     </refsect1>