Document ob_start change from php-src/pull/13087 (#4350)

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

Author: haszi

reference/outcontrol/functions/ob-start.xml

@@ -171,6 +171,12 @@
          </tbody>
         </tgroup>
        </informaltable>
+       <note>
+        <simpara>
+         Prior to PHP 8.4.0, the flags parameter could set
+         the <link linkend="outcontrol.constants.flags-returned-by-handler">output handler status flags</link> as well.
+        </simpara>
+       </note>
       </para>
      </listitem>
     </varlistentry>