--- a/src/pyams_utils/tales.py	Fri Jan 18 13:52:11 2019 +0100
+++ b/src/pyams_utils/tales.py	Fri Jan 18 13:57:58 2019 +0100
@@ -12,17 +12,13 @@
 
 __docformat__ = 'restructuredtext'
 
-
-# import standard library
 import re
 
-# import packages
 from chameleon.astutil import Symbol
 from chameleon.codegen import template
 from chameleon.tales import StringExpr
 from zope.contentprovider.tales import addTALNamespaceData
 
-# import interfaces
 from pyams_utils.interfaces.tales import ITALESExtension
 
 
@@ -48,6 +44,13 @@
     """TALES extension renderer
 
     See :ref:`tales` for complete description.
+
+    The requested extension can be called with our without arguments, like in
+    ${structure:tales:my_expression} or ${structure:tales:my_expression(arg1, arg2)}.
+    In the second form, arguments will be passed to the "render" method; arguments can be
+    static (like strings or integers), or can be variables defined into current template
+    context; other Python expressions including computations or functions calls are actually
+    not supported, but dotted syntax is supported to access inner attributes of variables.
     """
 
     def get_value(econtext, arg):