added some more information about unknown variable/attributes in the doc (closes #685)

doc/api.rst

@@ -60,6 +60,8 @@ You can also load and render the template in one fell swoop::
 
     echo $twig->render('index.html', array('the' => 'variables', 'go' => 'here'));
 
+.. _environment_options:
+
 Environment Options
 -------------------
 

doc/templates.rst

@@ -78,7 +78,9 @@ properties of a PHP object, or items of a PHP array), or the so-called
     variable but the print statement. If you access variables inside tags
     don't put the braces around.
 
-If a variable or attribute does not exist you will get back a ``null`` value.
+If a variable or attribute does not exist, you will get back a ``null`` value
+when the ``strict_variables`` option is set to ``false``, otherwise Twig will
+throw an error (see :ref:`environment options<environment_options>`).
 
 .. sidebar:: Implementation