Update striptags.rst

doc/filters/striptags.rst

@@ -12,4 +12,10 @@ by one space:
 
     Internally, Twig uses the PHP `strip_tags`_ function.
 
+    As a consequence, a parameter, ``$allowable_tags``, can be provided. In this example, the tags like ``<br/>``, ``<br>``, ``<p>`` and ``</p>`` will remain in the string:
+
+.. code-block:: jinja
+
+    {{ some_html|striptags('<br><p>') }}
+
 .. _`strip_tags`: http://php.net/strip_tags