Update spl_autoload_register

Add callback signature to parameter section.

Closes GH-629.

Author: thinkverse

reference/spl/functions/spl-autoload-register.xml

@@ -45,6 +45,10 @@
        If no parameter is provided, then the default implementation of
        <function>spl_autoload</function> will be registered.      
       </para>
+      <methodsynopsis>
+       <type>void</type><methodname><replaceable>callback</replaceable></methodname>
+       <methodparam><type>string</type><parameter>class_name</parameter></methodparam>
+      </methodsynopsis>
      </listitem>
     </varlistentry>
     <varlistentry>