Fixed #13116 -- Described scope of variables created by {% url ... as var %} syntax.

Thanks leif_p for the suggestion.

docs/ref/templates/builtins.txt

@@ -1053,11 +1053,13 @@ cause your site to display an error page.
 If you'd like to retrieve a URL without displaying it, you can use a slightly
 different call::
 
-
     {% url 'path.to.view' arg arg2 as the_url %}
 
     <a href="{{ the_url }}">I'm linking to {{ the_url }}</a>
 
+The scope of the variable created by the  ``as var`` syntax is the
+``{% block %}`` in which the ``{% url %}`` tag appears.
+
 This ``{% url ... as var %}`` syntax will *not* cause an error if the view is
 missing. In practice you'll use this to link to views that are optional::