Fill Memcached::__construct() parameters description

reference/memcached/memcached/construct.xml

@@ -40,16 +40,25 @@
     <varlistentry>
      <term><parameter>callback</parameter></term>
      <listitem>
-      <para>
-       <!-- TODO Document constructor params -->
-      </para>
+      <methodsynopsis>
+       <type>void</type><methodname><replaceable>callback</replaceable></methodname>
+       <methodparam><type>Memcached</type><parameter>memcached</parameter></methodparam>
+       <methodparam><type>string|null</type><parameter>persistent_id</parameter></methodparam>
+      </methodsynopsis>
+      <simpara>
+       The <parameter>callback</parameter> parameter is called when the
+       connection is established. It should be a valid PHP <type>callable</type>
+       that receive the <classname>Memcached</classname> object as its first
+       parameter, and <parameter>persistent_id</parameter> as its second.
+      </simpara>
      </listitem>
     </varlistentry>
     <varlistentry>
      <term><parameter>connection_str</parameter></term>
      <listitem>
       <para>
-       <!-- TODO Document constructor params -->
+       This parameter is used to pass additional connection options to
+       the memcache servers, such as a server weight in a cluster.
       </para>
      </listitem>
     </varlistentry>