Fixed #25051 -- Clarified return type of {% now %} tag.

docs/ref/templates/builtins.txt

@@ -814,8 +814,8 @@ This would display as "It is the 4th of September".
 
 
 You can also use the syntax ``{% now "Y" as current_year %}`` to store the
-output inside a variable. This is useful if you want to use ``{% now %}``
-inside a template tag like :ttag:`blocktrans` for example::
+output (as a string) inside a variable. This is useful if you want to use
+``{% now %}`` inside a template tag like :ttag:`blocktrans` for example::
 
     {% now "Y" as current_year %}
     {% blocktrans %}Copyright {{ current_year }}{% endblocktrans %}