Add warning about FILTER_SANITIZE_STRING tag stripping behaviour

Integrates note 118186

Author: Girgias

reference/filter/constants.xml

@@ -877,6 +877,12 @@
       use <function>htmlspecialchars</function> instead.
      </simpara>
     </warning>
+    <warning>
+     <simpara>
+      The way this filter strips tags is not equivalent to
+      <function>strip_tags</function>.
+     </simpara>
+    </warning>
    </listitem>
   </varlistentry>
   <varlistentry xml:id="constant.filter-sanitize-stripped">