Add a sentence from which PHP version PHP_OUTPUT_HANDLER_PROCESSED is available (#3587)

Closes GH-3384

Co-authored-by: haszi <[email protected]>

Author: haszi

reference/outcontrol/constants.xml

@@ -198,6 +198,7 @@
    <listitem>
     <para>
      Indicates that the output handler successfully processed the buffer.
+     Available as of PHP 8.4.0.
     </para>
    </listitem>
   </varlistentry>