fixed if tag documentation

doc/templates.rst

@@ -604,9 +604,8 @@ course this is not limited to commas:
 If
 ~~
 
-The ``if`` statement in Twig is comparable with the if statements of PHP. In the
-simplest form you can use it to test if a variable is defined, not empty or
-not false:
+The ``if`` statement in Twig is comparable with the if statements of PHP. In
+the simplest form you can use it to test if a variable is not empty:
 
 .. code-block:: jinja
 
@@ -618,6 +617,11 @@ not false:
       </ul>
     {% endif %}
 
+.. note::
+
+    If you want to test if the variable is defined, use ``if users is
+    defined`` instead.
+
 For multiple branches ``elseif`` and ``else`` can be used like in PHP. You can use
 more complex ``expressions`` there too: