Document Memcached::__construct() parameters

[cracksalad]

This is a follow up of #3873

The callback and connection_str parameters are not yet documented.

doc-en/reference/memcached/memcached/construct.xml

Lines 40 to 55 in 54eaf4e

	<varlistentry>
	<term><parameter>callback</parameter></term>
	<listitem>
	<para>
	<!-- TODO Document constructor params -->
	</para>
	</listitem>
	</varlistentry>
	<varlistentry>
	<term><parameter>connection_str</parameter></term>
	<listitem>
	<para>
	<!-- TODO Document constructor params -->
	</para>
	</listitem>
	</varlistentry>