Index: docs/templates_python.txt
===================================================================
--- docs/templates_python.txt	(revision 4750)
+++ docs/templates_python.txt	(working copy)
@@ -212,6 +212,9 @@
 tags, the variable will be interpreted as ``None``. Filters are always
 applied to invalid variables within these template tags.
 
+If TEMPLATE_STRING_IF_INVALID contains a ``'%s'``, it will be replaced with
+the name of the invalid variable.
+
 .. admonition:: For debug purposes only!
 
     While ``TEMPLATE_STRING_IF_INVALID`` can be a useful debugging tool,