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

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::