Add formal deprecation warning.

Author: Crell

reference/funchand/functions/get-defined-functions.xml

@@ -27,6 +27,7 @@
       Whether disabled functions should be excluded from the return value. This parameter has no effect
       since PHP 8.0.0 and is deprecated since 8.5.0.  It should not be used.
      </para>
+     &warn.deprecated.feature-8-5-0;
     </listitem>
    </varlistentry>
   </variablelist>