Expanded definition of empty

doc/tests/empty.rst

@@ -1,7 +1,7 @@
 ``empty``
 =========
 
-``empty`` checks if a variable is empty:
+``empty`` checks if a variable is an empty string, an empty array, an empty hash, exactly ``false``, exactly ``null``:
 
 .. code-block:: jinja
 
@@ -9,3 +9,6 @@
     {% if foo is empty %}
         ...
     {% endif %}
+
+.. _Countable: http://php.net/manual/en/class.countable.php
+.. _count: http://php.net/manual/en/function.count.php